month-view: rework handling of widget's hiding

Previously, we were hoping keeping a list of the widgets hidden by the
application, but the implementation did not work.
Now we keep a hash of those widgets hidden because of the overflow, when
a widget is not visible, and it's not in GcalMonthView::hidden_as_overflow
hash, then was hidden by the application.
month-view: add helper method
month-view: implement GcalView::get_by_uuid