GQueue: accept a NULL sibling for insert_before() and insert_after()

It simplifies a little bit some code that inserts data relative to a
GList location, that might be NULL for the tail of the queue. A NULL
sibling is probably less useful for insert_after(), so it's more for
consistency with insert_before().

https://bugzilla.gnome.org/show_bug.cgi?id=736620
Simplify code that uses g_queue_insert_before() and insert_after()

g_queue_insert_before() and g_queue_insert_after() now accept a NULL
sibling.

https://bugzilla.gnome.org/show_bug.cgi?id=736620