Cache created X11 cursors until program exit.

Each set_cursor was taking 0.5ms, due to libXcursor having to open, parse, and close
cursor theme files. Other toolkits likewise cache created cursors.


git-svn-id: http://seriss.com/public/fltk/fltk/branches/branch-1.3@10260 ea41ed52-d2ee-0310-a9c1-e6b18d33e121