calendar: Only allow closing events on the current day

The design calls for differentiating between dismissable reminders
and permanent events, based on whether the selected date is "today"
or some other day.

https://bugzilla.gnome.org/show_bug.cgi?id=744927

js/ui/calendar.js

@@ -1198,6 +1198,10 @@ const EventMessage = new Lang.Class({
                 title = title + ELLIPSIS_CHAR;
         }
         return title;
+    },
+
+    canClose: function() {
+        return _isToday(this._date);
     }
 });