Revert the revert in r10060, reinstating commit r10030.
The reason the code was problematic was because of r4589, where
a call to PostQuitMessage() snuck in. As we could not find a
valid reason for that PostQuitMessage(), it has been removed and
FLTK is now again properly responsive to WM_QUIT messages.


git-svn-id: http://seriss.com/public/fltk/fltk/branches/branch-1.3@10094 ea41ed52-d2ee-0310-a9c1-e6b18d33e121