Track block objects explicitly.

So that our picture always matches UDisks.  Otherwise we might forget
about blocks when they are unpublished, and will never publish them
again.

* Keep a hashtable in UlManager that tracks UDisks block objects.
* Turn UlBlock into a plain GObject and adjust all users.
* Update a newly created UlBlock from all existing volume groups to avoid
  a race with UDisks.
Don't support explicit publishing of GDBusObjects.

We don't need that anymore.
Remove duplicate typedef.
Trick UDisks into seeing thinly provisioned volumes.