Added new drag hover and drop states.
Merge pull request #391 from huwshimi/hover-states

Added new drag hover and drop states.

Added new drop states and highlighting when tokens are dragged over drop targets.

QA:
- drag two charms to the canvas
- open the machine view
- start dragging one of the units
- the 'Create new machine' should appear in the header with new styling
- hover the unit over the drop area
- the background should go grey
- drop the unit on the header and click create in the constraints form
- click on the 'new0' machine
- start dragging the second unit
- the 'Create new machine' and 'Create new container' headers' targets should appear
- the machine and container tokens should show the 'Add to machine new0' and 'Add to container Bare metal' respectively
- hovering the unit over the headers and tokens should turn them grey
- dropping the unit on a header or token should make the drop areas disappear