Hide drop targets when unit is dropped on a header

This branch fixes a bug where dropping a unit on a header would leave the drop targets visible.

https://bugs.launchpad.net/juju-gui/+bug/1326240

QA:
- drag a service to the canvas
- open the machine view
- drag a unit to "Create new machine"
- the header should return to "Environment" (with the machine count etc.) and a new machine should be created

Dropping a unit on the containers header should also return the headers to the correct states.