Merge lp://qastaging/~mterry/libdbusmenu/unref-gtkmenuitems into lp://qastaging/libdbusmenu/0.5

Another result of my investigations into bug 835646. I don't think this is the last leak, but it should help a bit.

GtkMenuItems weren't necessarily being finalized, because we would still hold a reference to them.

dbusmenu_gtkclient_newitem_base would always call g_object_ref_sink, owning the initial floating ref. Then the menu item would often be added to a menu, which would call ref_sink again, adding a normal ref (total of 2 now).

When the corresponding DbusmenuMenuitem would die, we used to just be calling gtk_widget_destroy, which removes the item from any containers and does some other cleanups. Being unparented would reduce the ref count to 1, where it would remain.

So I've added another g_object_unref call to match our initial ref_sink. And some comments to explain why.