js: Use EVENT_PROPAGATE/EVENT_STOP constants in event handlers

Just as SOURCE_CONTINUE/SOURCE_REMOVE in source functions, these
constants increase code clarity over plain true/false.

https://bugzilla.gnome.org/show_bug.cgi?id=719567

js/ui/calendar.js

@@ -497,6 +497,7 @@ const Calendar = new Lang.Class({
             this._onNextMonthButtonClicked();
             break;
         }
+        return Clutter.EVENT_PROPAGATE;
     },
 
     _onPrevMonthButtonClicked: function() {