Add drop states to the machine view headers.
Global units model list.
Merge pull request #291 from huwshimi/header-drop-states

Adding in the drop states in the machine view headers. These states are not currently hooked up to anything, but are ready for when drag and drop is implemented.
Merge pull request #282 from frankban/unit-list

The db.units lazy model list includes all known units.
Also implement helper methods to add/remove units, and filter them by machine/container.

I was not able to properly live test this, but I am still proposing the code in order to unblock other branches.
For this reason please be very meticulous when QAing the branch: all the units-related GUI parts (showing units in the inspector, adding/removing them) should be tested for regressions, in both sandbox and real environments.

This does not include changes in the machine panel (wire service unit tokens etc.): the diff is already too long (my apologies). I changed my card description and created another one in "Ready to code" to reflect this.