gui/event-widget: Use more bindings in the UI file

Instead of manually connecting to signals and binding to properties
in code.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/758>

src/gui/gcal-event-widget.blp

@@ -11,6 +11,9 @@ template $GcalEventWidget: Widget {
   }
 
   DragSource drag_source {
+    propagation-phase: bind $read_only_to_propagation_phase_cb(
+      template.event as <$GcalEvent>.calendar as <$GcalCalendar>.read-only
+      );
     prepare => $on_drag_source_prepare_cb(template) not-swapped;
     begin => $on_drag_source_begin_cb(template) not-swapped;
   }
@@ -64,7 +67,9 @@ template $GcalEventWidget: Widget {
     Box horizontal_box {
       spacing: 4;
 
-      $GcalFadingLabel summary_label {}
+      $GcalFadingLabel summary_label {
+        label: bind template.event as <$GcalEvent>.summary;
+      }
 
       Label timestamp_label {
         halign: end;