The subscriber view was an unecessary layer and only
added more complication than it actually helped.
This commit merges GcalSubscriberView and GcalMonthView
into a single class.
When deleting events with recurrence, we want to hide the event
widgets according to the modification type. So that selecting
e.g. subsequent in the modification type dialog hides the event,
and the subsequent ones, truly reflecting the operation that will
happen.
Currently, however, the views don't do that, and only hide the
event that was asked to be deleted.
Fix that by passing the object modification type, and adapting the
views to take that into account.
The current implementation is deficient in many ways, the
worst one being the it relying on GObject semi-synchronous
property emission.
This issue was exposed by the Week view code, where the active
date was updated, but delayed to reach the Week header and some
events were being added ~before~ the active date changed.
With this work, I was also able to figure out a bunch of code
that relied on this (i.e. luck) to work.