garray: Add g_ptr_array_steal_index*() functions

These make it easy to steal elements from pointer arrays without having
the array’s GDestroyNotify called on them, similarly to what’s possible
with g_hash_table_steal().

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

https://bugzilla.gnome.org/show_bug.cgi?id=795376