Prevent invalid drag/drops

We don't allow dragging and dropping a chidlren/servergroup
selection, unless those children belong to the servergroups.
To avoid this we remove all children belonging to server
groups in the list, and if any children remain we cancel
the drop.
Fix the last few drag/drop bugs everything should be good now

Still need to make a popup for when you try to take a group's
last server though.