Rename some Fl_Flex methods for FLTK compliance (#594)

Change some method names to comply with FLTK style as discussed
in fltk.coredev, thread "Fl_Flex method name question".

* Rename Fl_Flex::margins(...) to Fl_Flex::margin(...)
  (use singular form for all margin related methods)

* Remove Fl_Flex::setSize() and isSetSize() "backwards compatibility" methods

* Rename Fl_Flex::set_size(...) to fixed(...)

Note: the latter affects existing (pre-release) fluid (.fl) files: you may
  want to edit and replace "set_size_tuples" with "fixed_size_tuples"

examples/howto-flex-simple.cxx

@@ -49,7 +49,7 @@ int main(int argc, char **argv) {
 
   // set gap between adjacent buttons and extra spacing (invisible box size)
   flex.gap(10);
-  flex.set_size(bx, 30); // total 50: 2 * gap + 30
+  flex.fixed(bx, 30); // total 50: 2 * gap + 30
 
   // end() groups
   flex.end();