Patch for user_data

[Andreas Held]

Hi

When refactoring the callback implementation for pyFltk, the Python
bindings for fltk, I noticed that the Fl_Callback_User_Data concept is
not yet supported by classes Fl_File_Chooser and Fl_Tree_Item. As this
would simplify our life with pyFltk, I have tried to come up with a
patch for the implementation in fltk V1.5. You can see my attempts here:
https://github.com/AndreasHeld62/fltk/tree/user_data_patch

If this is of interest and my approach is acceptable, then I would be
more than happy to create a merge request.

Best regards

Andreas