More on memory leaks

Since I'm doing gtk+ for a very long time there some common problems like not freeing GList retrieved by gtk_container_get_children(). Next commit(s) is going to fix it.

Further some strings may not be freed correctly. AgsFile uses libxml2 and it's not evident if attribute values are freed correctly. To target string memory leaks would be next big thing.

API of Advanced Gtk+ Sequencer may adopt the behaviour of gtk_container_get_children() for retrival of any child objects.