- remove build option FLTK_USE_KDIALOG replaced by an Fl::option() option
- new run-time option OPTION_FNFC_USES_KDIALOG
- make options OPTION_FNFC_USES_KDIALOG and OPTION_FNFC_USES_ZENITY
false by default
- add mention of new program fltk-options in the doc of Fl::option()
- change logic of choice of the native file chooser under X11/Wayland:
the zenity and kdialog choosers are opt-in; otherwise the GTK file chooser
is used, unless opted out with OPTION_FNFC_USES_GTK
- document that zenity may be interesting for sandboxed apps
- document that both zenity and kdialog make member functions
Fl_Native_File_Chooser::filter_value() inoperable