making Group/User dropdown menu configurable, close #11120

This PR covers [ticket 11120](http://trac.openmicroscopy.org.uk/ome/ticket/11120).
Additional configuration allows to manipulate drop down menu, as shown on the picture
![firefoxscreensnapz004](https://f.cloud.github.com/assets/1065155/653887/7143601c-d4e8-11e2-86fd-238a54a7d70b.jpg).