ScrolledWindow handles PgUp/PgDown in bubble phase to trigger scrolling.
Which leads to inconsistent behaviour: scrolling when an event is focused,
and moving to the previous/next week when not. Explicitly handling the
shortcuts with a ShortcutController in capture phase makes it consistently
navigate to previous/next week.
Fixes#1409
Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/618>
