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/misc/history.js

@@ -89,7 +89,7 @@ const HistoryManager = new Lang.Class({
         } else if (symbol == Clutter.KEY_Down) {
             return this._setNextItem(entry.get_text());
         }
-        return false;
+        return Clutter.EVENT_PROPAGATE;
     },
 
     _indexChanged: function() {