restore the new api after the 4.8 merge

the new api was removed from 4.8 branch, merging removed it from master, add it again, so merging 4.8 into master will work (at least until the first cherrypick ;)

declarativeimports/plasmacomponents/qmenu.h

@@ -52,6 +52,8 @@ public:
     void showMenu(int x, int y);
     Q_INVOKABLE void open();
     Q_INVOKABLE void close();
+    Q_INVOKABLE void clearMenuItems();
+    Q_INVOKABLE void addMenuItem(const QString &text);
 
 Q_SIGNALS:
     void statusChanged();