* Bundle Fl_Text_Buffer from FLTK 1.1, renamed as Fl_Text_Buffer_mod.
* Bundle Fl_Text_{Buffer,Display,Editor} from FLTK 1.3, renamed as
above. Apply most of Fl_Text_Display->Fl_Text_Display_mod changes
to the 1.3 Fl_Text_Display.
* Detect FLTK API at configure time and define macros to determine
version of the *Text* files will be built.