glib: Fix native GLib warnings when disposing SourceProxy objects

When an instance of SourceProxy was finalized, we would try to remove
the corresponding source, even if it was already removed. This now
causes native GLib to print out warnings because it can't find the
source ID.

Now Source.Remove only calls g_source_remove if we really had a handler
registered for the ID we're removing.
Check that source_handlers contains the tag.
Include Cody's native library changes in August 2014