This allows GSource implementors to safely clear any other references to
the GSource while the GSource is still valid, unlike when doing the same
from the finalize function.

After the dispose function has run, it is valid for the reference count
of the GSource to be > 0 again to allow the case where another thread in
the mean-time got access to the GSource reference before the dispose
function was called.

This allows fixing a thread-safety issue in the GCancellable, GstBus and
various other GSource implementations.