This is part 2 of the final fix for a previous name clash on case insensitive file systems (fl_string.h vs. Fl_String.H).
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.
This class will be used in fl_input_str() and fl_password_str(). Todo: add missing documentation ...
