common/date-chooser: Use GcalGizmo

GcalGizmo allows us to handle focus within child widgets
much more cleanly, as it no longer requires us to override the
ancestor and re-iterating through children.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/760>

src/gui/common/gcal-date-chooser.blp

@@ -64,15 +64,19 @@ template $GcalDateChooser: Adw.Bin {
       }
     }
 
-    Grid grid {
-      row-homogeneous: true;
-      column-homogeneous: true;
+    $GcalGizmo gizmo {
+      layout-manager: BinLayout {};
 
       layout {
         column: 0;
         column-span: 2;
         row: 1;
       }
+
+      Grid grid {
+        row-homogeneous: true;
+        column-homogeneous: true;
+      }
     }
   }
 }