Revert "dialog-private: replace QJSValue with QQmlV4Handle in js functions"

This reverts commit b365471f0abc79f08bf0d852aea3be0a601c6901.

We need to be able to change the internal API in QtQml, which is difficult if
other modules are using it. It's much easier here to simply use QJSValue.

Change-Id: Ibbc078d8deffcf4f5a96a4cfe50c836d6653d666
Reviewed-by: Liang Qi <liang.qi@digia.com>