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"
