window: Go back/forth views using hardware buttons

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>

src/gui/gcal-window.blp

@@ -105,6 +105,16 @@ template $GcalWindow: Adw.ApplicationWindow {
     }
   }
 
+  GestureClick {
+    button: 8; // Hardware back button.
+    pressed => $on_back_button_pressed_cb(template) not-swapped;
+  }
+
+  GestureClick {
+    button: 9; // Hardware forward button.
+    pressed => $on_forward_button_pressed_cb(template) not-swapped;
+  }
+
   Adw.ToastOverlay overlay {
     $GcalDropOverlay drop_overlay {
       overlayed: Adw.StatusPage drop_status_page {