- add true unittest and Fl_String testing
- interface and printout are similar to gtest
without requiring external linkage.
just run `unittest --core`.
- new Fl_String API
- extended API to fl_input_str and fl_password_str
- co-authored-by: Albrecht Schlosser <albrechts.fltk@online.de>
Also
- fix "\todo Unicode?" in Fl_Text_Display::display_insert() after verification
- fix "\todo [Internal] The user_data value must be implemented using \p fl_intptr_t"
because that's now in place.
- remove static variable 'string_count'
- improve and document methods debug() and hexdump()
This commit does not resolve issue 601 but removes some development
leftovers.
Methods debug() and hexdump() left for debugging as requested.
The previous name existed already with different case (fl_string.h)
in the FL folder which broke the build on macOS and Windows.
This may be a temporary fix - until I find a better way.
