"Shift to parent" does not make any sense for collections since they don't
have parenting like objects. This commit just adds a simple check for
whether the first drag ID is a collection.

Checking only the first ID works here because only ID types that match
that of the "dragged" (under the mouse) element are moved as part of the
drag. I figured looping through all of the IDs here would be discouraged,
a more complete check would be easy to add if it isn't.