Editor should be able to select multiple slots and create a new group containing those slots

[fechan]

The user should be able to select 1+ slots, drag them from its existing group, and form a new group in its current machine.

Nothing should happen if this would cause the existing group to be empty and the new group would have the same slots as the old one.

[fechan]

You can now drag a single slot and form a new group.

I'd like a multiple select thing at some point, but this should do for release 1.