Add g_hash_table_get_key_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

docs/reference/glib/glib-sections.txt

@@ -2296,6 +2296,7 @@ g_hash_table_remove_all
 g_hash_table_steal_all
 g_hash_table_get_keys
 g_hash_table_get_values
+g_hash_table_get_key_array
 GHRFunc
 g_hash_table_freeze
 g_hash_table_thaw