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/ibusCandidatePopup.js

@@ -32,6 +32,7 @@ const CandidateArea = new Lang.Class({
             let j = i;
             box.connect('button-release-event', Lang.bind(this, function(actor, event) {
                 this.emit('candidate-clicked', j, event.get_button(), event.get_state());
+                return Clutter.EVENT_PROPAGATE;
             }));
         }