Add the initial version of class Fl_Scheme

This basic version of class Fl_Scheme contains only static methods that
are needed for Fl_Scheme_Choice and further extensions.

FL/Fl_Scheme.H

@@ -0,0 +1,64 @@
+//
+// Scheme header for the Fast Light Tool Kit (FLTK).
+//
+// Copyright 2022-2023 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_Fl_Scheme_H_
+#define FL_Fl_Scheme_H_
+
+#include <FL/Fl.H>
+
+class Fl_Scheme {
+
+private:
+
+  static const char **names_;           // registered scheme names
+  static int num_schemes_;              // number of registered schemes
+  static int alloc_size_;               // number of allocated scheme name entries
+
+protected:
+
+  // const char *name_;                 // the scheme's name
+
+  // protected constructor - not yet implemented
+  // Fl_Scheme(const char *name);
+
+public:
+
+  // Static methods.
+
+  // Some of these methods will replace the scheme related methods of class Fl,
+  // for instance Fl::scheme() and Fl::is_scheme().
+  // Backwards compatibility must be kept though.
+
+  static const char **names();
+
+  /**
+    Return the number of currently registered schemes.
+
+    \return  Number of registered schemes.
+  */
+  static int num_schemes() {
+    if (!names_) names(); // force initialization
+    return num_schemes_;
+  }
+
+  // Adding a scheme name must be a public static method in FLTK 1.4.0.
+  // This will later be protected or replaced by another method name.
+
+  static int add_scheme_name(const char *name);
+
+}; // class Fl_Scheme
+
+#endif // FL_Fl_Scheme_H_