Update version numbers to 1.5.0

ANNOUNCEMENT

CHANGES_1.4.txt

CMakeLists.txt

FL/Enumerations.H

FL/Fl_File_Chooser.H

FL/Fl_Help_Dialog.H

README.md

README.txt

documentation/src/index.dox

documentation/src/preface.dox

examples/fluid-callback.fl

fltk-options/fltk-options.plist

fltk_version.dat

fluid/about_panel.cxx

fluid/about_panel.fl

fluid/about_panel.h

fluid/codeview_panel.cxx

fluid/codeview_panel.fl

fluid/codeview_panel.h

fluid/documentation/src/index.dox