* Adding Fluid support for Fl_Flex, margins, and gap. * Fluid Fl_Flex mostly working. Fixed in Fl_Flex::set_size(). * Fluid Flex live mode works, interactive dragging works * Fluid Flex: adding check box for fixed children * Fluid Flex: visual flexibility indicator in guides. * Fluid Flex: bug in generated code. * Fix formatting * Fixing Tooltip, fixing resize issue PR #518. * Removing unused variables. Co-authored-by: Albrecht Schlosser <albrechts.fltk@online.de>