Only allow one unit token to be in placement mode

To prevent the list of machines and containers getting outdated by starting the move form on multiple unit tokens this branch makes it so you can only show the form on once token at once.

See: https://bugs.launchpad.net/juju-gui/+bug/1329224

QA:
- drag several charms to the canvas
- open the machine view
- click the 'move to' icon on one of the tokens
- now click on a different 'move to' icon
- the second token should now display the placement form and the first token should be reset to its initial state
- subsequent clicks on 'move to' icons should continue to only display the form on the last clicked token