MSBuild: fix the solution build (except audit and sample projects)

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.
gio: Delete DBusInterfaceVTable class

It was never included in the build, and never modified since it was
first created.