Replace deprecated Q_FLAGS macro

Q_FLAGS is deprecated, and is replaced by the more powerful Q_FLAG macro.

Anyhow, Q_FLAG and Q_FLAGS only makes sense if used inside
a QObject or Q_GADGET. The patch therefore makes QWebEngineContextMenuData
a Q_GADGET.

Change-Id: Icccb373846a645272a7bf29397b930ae80be9b85
Reviewed-by: Jüri Valdmann <juri.valdmann@qt.io>
Reviewed-by: Valentin Fokin <fokinv@inf.u-szeged.hu>
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>