Global units model 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.