This commit makes it possible to build any project of the gtk-sharp.sln
from an IDE (except audit and sample projects, which require a bit more
work).
This doesn't mean that autotools is deprecated, but just that it is more
comfortable to use an IDE when working on gtk-sharp because it will
offer better auto-completion, and will stop highlight misleading
semantic errors, from now on.
The Group property for all the Radio* classes (RadioAction, RadioButton,
RadioMenuItem and RadioToolButton) was exposed as a GLib.SList,
with its elements untyped.
We now hide the various Group properties and have custom code to handle
them correctly.