Updated CHANGES file with ABI mods.

Also: added release dates for all releases I could easily find copies of.
      To determine release dates, I used e.g.

      tar tvfz /net/software/fltk/fltk-1.1.1-source.tar.gz | sort --key=4 | tail

      ..the date stamps on the directories at the bottom of the report indicate
      the date the tar file was created.



git-svn-id: http://seriss.com/public/fltk/fltk/branches/branch-1.3@10017 ea41ed52-d2ee-0310-a9c1-e6b18d33e121