Add a GTK#3 Gui project

This PR adds a new  subproject named Mono.Addins.GuiGtk3, which is simply the Gui subproject ported to GTK#3. This is done as part of the #monohack 2013 hackfest effort to port apps to GTK#3, as many of the apps depend on Mono.Addins.Gui. Basically they can now pull in GuiGtk3 instead of Gui, and the API will be identical. (While those using GTK#2 can continue doing so...)

The add-in UI is slightly less pretty than the original and could use a fair bit of polish, but it should be fully functional. There has also been added a Tests project that uses GuiGtk3 to open a GUI for testing purposes.

The makefile setup is altered to allow conditional building of GTK#3 support, which defaults to off since it is not so common yet.