Refresh list of unplaced units on DB updates.
Merge pull request #305 from kadams54/unplaced-units-render

### QA

With `/:flags:/il/mv/`, switch to the machine view. Select a charm in the browser and then click on "Add to my canvas". When the details panel closes, you should see the unplaced service sitting in the Unplaced Units column.
Made the unplaced unit token draggable
service unit token now fires events to notify the machine panel view that it's being dragged or not
Makes the container token into a drop target for the unplaced unit token
Moved the drop target methods into a shared extension file
Added tests for the unplaced unit draggability
Renamed the drop extension to be better representative of where it works.
Dragging an unplaced unit now turns the column headers into drop targets
If the user hasn't selected a machine then they can only drop on the machine column
Added tests for the drop target UI
Fixed conflict issues