For QWebEnginePage users to be able to make custom context menu or
extend the default context menu, they need information about the
context.

This patch adds a QWebEngineContextMenuData class that contains
contextual information when a context menu is being requested.

This is also means we can finally generate a custom context-menu in
the demobrowser the same way we did in QtWebKit.

Task-number: QTBUG-51079
Change-Id: I695b8cbf648bdc7f119b0ed51ab685cf2f8de8e4
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@theqtcompany.com>