Now that we keep precise control over GcalEvent instances, and
can use simple pointer comparisons everywhere, GcalMonthView
implemented its update function as a simple remove + add pair.

The problem is that we're not passing the old event instance,
so removals never actually happen, since the new instance is
not yet available to subscribers.

Pass old event in additio to the new one, and use it where it
makes sense.

Closes #1073