gmain: Allow GSource methods to be called from a finalize() callback

Temporarily increase the ref count of a GSource to 1 while calling its
finalize() callback, so that the finalize() implementation can call
GSource methods (like g_source_set_ready_time()) without causing
critical warnings. It’s safe to call those methods at this point, as the
source has been destroyed, but nothing has been freed.

This is an indirect way of fixing a race between GCancellable and
GCancellableSource, whereby the GCancellable::cancelled callback for the
GCancellableSource is not disconnected until the GCancellableSource’s
finalize() function is called. Previously, this meant there was a window
in which the GCancellableSource’s ref count was 0, but the ::cancelled
callback was still connected, and could legitimately be called as a
result of another thread calling g_cancellable_cancel() on the
GCancellable. The callback calls g_source_set_ready_time() on the
GSource, and there’s no thread-safe way of checking whether ...