Small fixes following ReplaceGtkAction merge

So I've carried on reading through the changes from the ReplaceGtkAction branch merge. I have mainly focussed on finding small local memory leaks and similar issues (I hope that one day I will know enough about Thunar to be able to usefully review the substance of changes like these!). This branch contains fixes for the few issues I found in thunar-launcher.c and thunar-window.c (plus one fix in thunar-menu.c, which I have not read properly yet).

In particular several of the fixes consist of adding calls to g_list_free to match calls to gtk_container_get_children, as this method is transfer-container.

Thanks!