Add coordinates and layout section to user manual
add section to user manual to clarify the use of window-relative
coordinates in both Fl_Group and Fl_Window containers, and include
brief descriptions of current layout manager widgets in one place.
add test/coordinates.cxx, test/wizard.cxx and related screenshots
under documentation/src.
update CMakeLists.txt, Makefile and .gitignore for new files.
Co-authored-by: Albrecht Schlosser <albrechts.fltk@online.de>
engelsman