[Gtkradiant] gtk2 version

[Timothee Besset]

Looking back at the work Spog has done on the gtk2 version (see branch
port_gtk2_20030307), I am thinking that we should switch the 1.3 release
to use gtk2 soon, and not wait further for 1.3 stabilization / 1.4
release.

>From what X-Man says, the gtk2 build incidentally fixes some of the OSX
issues. I don't see any particular reason it would work badly on
Linux, and from testing feedback it's working fine on win32.

The main thing to do on the gtk2 branch before merging it back into trunk
is to update the win32 setup to distribute the correct DLLs. It is likely
that the Linux setup will need to be adapted, specially it will need
updated build system for ABI compatibility of the distribution.

Before we start releasing Gtk2 builds though, I'd like to have another
release with JKII/SoF2 support so we can get more feedback on that and
potentially more fixes:
http://zerowing.idsoftware.com/bugzilla/show_bug.cgi?id=781

It would be good to include Raven's grouping/ghosting code too:
http://zerowing.idsoftware.com/bugzilla/show_bug.cgi?id=764
I'm thinking that maybe some things in there are not compatible with the
gtk2 changes (though that may be unlikely)

TTimo