Sprinkle debug logs, improve URI to URN monitoring, some very initial UI foundations, etc.

Another batch of patches for #29 (closed).

Importing content from removable devices is going to heavily use the ItemManager infrastructure for monitoring the Tracker database for the addition of a new URI or updates to an existing URI. The existing code proved to be fragile and needs some hardening to deal with a lot of I/O.

Some basic UI work. We need a spinner to indicate content being added to a collection, and a custom ModelButton for showing devices and albums.

The app.search-* will now be used to select devices, so should be handled in a more general location than BaseModel.