Add Fl_Flex widget from Karsten Pedersen (issue #255)

This work is based on the repository and latest commit:
  https://github.com/osen/FL_Flex.git

  commit 36e4ed75a00daac825b87e81295818b4650991f5
  Author: Karsten Pedersen <...>
  Date:   Fri Apr 23 12:06:16 2021 +0000

    Added Fltk (LGPL) license.

This widget is similar to Fl_Pack and supports either one row or one
column of widgets but has some more features. Test and demo programs
are included:

  test/flex_login.cxx: simple "login window" demo program
  test/flex_demo.cxx:  slightly more complex demo program

The original demo programs can still be compiled and built with
the new widget provided you '#include <FL/Fl_Flex.H>'.
Backwards compatible methods are included (except debug()).

The original widget has been modified to match FLTK standards and
enhanced in several ways, including:

- support box frames
- add HORIZONTAL and VERTICAL enum values (as in Fl_Pack)
- add horizontal() method (as in Fl_Pack)
- use type() rather than internal 'direction' variable
- add standard widget constructor (x, y, w, h, label)
- add margin and gap accessors rather than hard coding constants
- improve test and demo programs
- add documentation
- replace <vector> with array as required by FLTK CMP
- rename camelCase method names, keeping old names for compatibility:
  - change 'setSize(Fl_Widget*, int)'   to 'set_size(Fl_Widget*, int)'
  - change 'bool isSetSize(Fl_Widget*)' to 'int set_size(Fl_Widget*)'
- remove debug() method
- add a way to "unset" fixed size: set_size(Fl_Widget *, 0)
- add layout() method to force recalculation of children
- unify resizeRow() and resizeCol() methods to avoid code duplication
- improve widget size calculation.

FL/Fl_Flex.H

@@ -0,0 +1,161 @@
+//
+// Fl_Flex widget header file for the Fast Light Tool Kit (FLTK).
+//
+// Copyright 2020 by Karsten Pedersen
+// Copyright 2022 by Bill Spitzak and others.
+//
+// This library is free software. Distribution and use rights are outlined in
+// the file "COPYING" which should have been included with this file.  If this
+// file is missing or damaged, see the license at:
+//
+//     https://www.fltk.org/COPYING.php
+//
+// Please see the following page on how to report bugs and issues:
+//
+//     https://www.fltk.org/bugs.php
+//
+
+#ifndef Fl_Flex_H
+#define Fl_Flex_H
+
+#include <FL/Fl_Group.H>
+
+class FL_EXPORT Fl_Flex : public Fl_Group {
+
+  int margin_;
+  int gap_;
+  int set_size_size_;
+  int set_size_alloc_;
+  Fl_Widget **set_size_;
+
+public:
+
+  enum { // values for type(int)
+    VERTICAL   = 0,     ///< vertical layout (one column)
+    HORIZONTAL = 1,     ///< horizontal layout (one row)
+    COLUMN     = 0,     ///< alias for VERTICAL
+    ROW        = 1      ///< alias for HORIZONTAL
+  };
+
+  // FLTK standard constructor
+  Fl_Flex(int X, int Y, int W, int H, const char *L = 0);
+
+  // original Fl_Flex constructors:
+  // backwards compatible if direction *names* { ROW | COLUMN } are used
+
+  Fl_Flex(int direction);
+  Fl_Flex(int w, int h, int direction);
+  Fl_Flex(int x, int y, int w, int h, int direction);
+
+  virtual ~Fl_Flex();
+
+  virtual void end();
+  virtual void resize(int x, int y, int w, int h);
+
+  void set_size(Fl_Widget *w, int size);
+  int set_size(Fl_Widget *w);
+
+protected:
+
+  void init(int t = VERTICAL);
+
+  int alloc_size(int size);
+
+public:
+
+  /** Return the margin size of the widget.
+    \return margin size.
+  */
+  int margin() { return margin_; }
+
+  /** Set the margin and optionally the gap size of the widget.
+    This method can be used to set both the margin and the gap size.
+
+    If you don't use the second parameter \p g or supply a negative value
+    the gap size is not changed.
+
+    The margin is some free space inside the widget border \b around all child
+    widgets. It has the same size at all four edges of the Fl_Flex widget.
+
+    The gap size \p g is some free space \b between child widgets.
+
+    \param[in]    m   margin size, must be \>= 0
+    \param[in]    g   gap size, must be \>= 0, or will be ignored (if negative)
+
+    \see gap(int)
+  */
+
+  void margin(int m, int g = -1) {
+    margin_ = m < 0 ? 0 : m;
+    if (g >= 0)
+      gap_ = g;
+  }
+
+  /** Return the gap size of the widget.
+    \return gap size between all child widgets.
+  */
+  int gap() { return gap_; }
+
+  /**
+    Set the gap size of the widget.
+
+    The gap size is some free space \b between child widgets.
+    The size must be \>= 0. Negative values are clamped to 0.
+
+    \param[in]  g   gap size
+  */
+  void gap(int g) {
+    gap_ = g < 0 ? 0 : g;
+  }
+
+  /** Returns non-zero (true) if Fl_Flex alignment is horizontal (row mode).
+
+    \returns non-zero if Fl_Flex alignment is horizontal
+    \retval 1   if type() == Fl_Flex::HORIZONTAL
+    \retval 0   if type() == Fl_Flex::VERTICAL
+
+    See class Fl_Flex documentation for details.
+  */
+  int horizontal() const {
+    return type() == Fl_Flex::HORIZONTAL ? 1 : 0;
+  }
+
+  /**
+    Calculates the layout of the widget and redraws it.
+
+    If you change widgets in the Fl_Flex container you should call this method
+    to force recalculation of child widget sizes and positions. This can be
+    useful (necessary) if you hide(), show(), add() or remove() children.
+
+    This method also calls redraw() on the Fl_Flex widget.
+  */
+  void layout() {
+    resize(x(), y(), w(), h());
+    redraw();
+  }
+
+#if (1)
+
+  // Additional methods for backwards compatibility with "original" Fl_Flex widget
+
+  /**
+    Deprecated.
+    \deprecated Please use set_size(Fl_Widget *) instead.
+  */
+  bool isSetSize(Fl_Widget *w) {
+    return (bool)set_size(w);
+  }
+
+  /**
+    Set the horizontal or vertical size of a child widget.
+    \deprecated Please use set_size(Fl_Widget *, int) instead.
+  */
+  void setSize(Fl_Widget *w, int size) {
+    set_size(w, size);
+  }
+
+#endif
+
+};
+
+#endif // Fl_Flex_H