Add g_hash_table_get_keys_as_array()

Returns a %NULL-terminated array of the keys of a hashtable.

In the case that the hash table has strings for keys, this is actually a
gchar**.

https://bugzilla.gnome.org/show_bug.cgi?id=710964
tests: test g_hash_table_get_keys_as_array()

https://bugzilla.gnome.org/show_bug.cgi?id=710964