added possibility to handle nested overlays separately

[aschiffmann]

trafficstars

I noticed that that the plugin cannot handle nested overlays properly: After adding overlays to several nested containers and removing only one overlay from a parent container, all other overlays of all nested subcontainers will be removed, too.

I modified the behaviour, so that only the overlay of the specified container is removed, and those of the nested subcontainer will be kept. So now overlays of sub-containers can (and have to) be removed separately.