Fix dropping a transition endpoint on a state.

The drop target is the DropArea element, which is a child somewhere below
the state top-level element. So, walk up the hierarchy until we find what
we are looking for.

Change-Id: I9c0779b3cf5b4c329ad53c52658339aa874773c2
Reviewed-on: https://codereview.kdab.com/10953
Reviewed-by: Continuous Integration <build@kdab.com>
Reviewed-by: Sebastian Sauer <sebastian.sauer@kdab.com>