ghash: Add g_hash_table_steal_extended()

This is a combination of g_hash_table_lookup_extended() and
g_hash_table_steal(), so that users can combine the two to reduce code
and eliminate a pointless second hash table lookup by
g_hash_table_steal().

Signed-off-by: Philip Withnall <withnall@endlessm.com>

https://bugzilla.gnome.org/show_bug.cgi?id=795302