gmain: Unref child sources when finalising a GSource

If a GSource is created, *not* attached to a GMainContext, and then has
child sources added, dropping the last reference to the parent GSource
will leak its references to its child sources. Currently, child sources
are only unreffed when g_source_destroy() is called on the parent.

https://bugzilla.gnome.org/show_bug.cgi?id=737338