calendar-list: move previous/next week with PgUp/PgDown via ShortcutController

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>

src/gui/gcal-calendar-list.blp

@@ -15,6 +15,21 @@ template $GcalCalendarList: Adw.Bin {
         hexpand: true;
         selection-mode: none;
         row-activated => $on_listbox_row_activated_cb(template) not-swapped;
+        // Allow Page-Up/Page-Down to consistently switch months/weeks,
+        // regardless of whether the sidebar's calendars listbox has focus.
+        ShortcutController {
+          propagation-phase: capture;
+
+          Shortcut {
+            trigger: "Page_Up";
+            action: "action(win.previous-date)";
+          }
+
+          Shortcut {
+            trigger: "Page_Down";
+            action: "action(win.next-date)";
+          }
+        }
 
         styles [
           "navigation-sidebar",