Refactor `on_window_previous_date_activated_cb` and `on_window_next_date_activated_cb` into separate methods for going to the prev/next view, to be used in the callbacks for both the forward/back buttons in the UI, and for the `Gtk.GestureClick` callbacks that listens for the hardware buttons to be pressed. Closes: https://gitlab.gnome.org/GNOME/gnome-calendar/-/issues/1494 Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/682>