With the latest GLib, g_source_remove() complains about not finding
the timeout source with the given ID here, since it was already
destroyed by returning FALSE from the timeout callback. Also return
FALSE from the bus watches when we don't want to be called any more.
Make appsrc not set caps on buffers when its own caps is NULL.
This avoids calling make_metadata_writable on all buffers and
prevents losing buffer caps in case we are not replacing it
with something meaningful.
https://bugzilla.gnome.org/show_bug.cgi?id=630353