Currently, clicking on a calendar in the sidebar's listbox traps
the keyboard focus. As a result, pressing PageUp or PageDown
scrolls the listbox instead of switching the date.
Similar to commit fbc18174 for the week view, this adds a
capture-phase ShortcutController to the calendar listbox. This
ensures that PgUp and PgDown consistently switch month or week
regardless of whether a calendar item is focused.
Fixes#1593
Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/771>
