When enabling or disabling a calendar through the GcalCalendarButton
popover, the user loses the ability to click right after. This bug was
caused by the update mechanism, triggered by the calendar-changed signal,
which would delete then create a brand new GtkWidget for the row.
To fix this, I replaced the manual signal-based create, update and
remove mechanisms with a binding between the GtkListBox and the calendar
GListModel.
