Add QWebChannel setter and getter to QWebEnginePage

This allows using the WebChannel integration from the widgets API.
Contains documentation, the underlying API for transport is tested
by the qml tests already.

Change-Id: If5520eddf5d1a9c917df890960e042f53baa42ce
Reviewed-by: Andras Becsi <andras.becsi@theqtcompany.com>