Implement QWebEnginePage::action for basic navigation.

This imports code from qwebpage.cpp and QWebPageAdapter.cpp of WebKit
into qwebenginepage.cpp and thus also restrict the license accordingly.

Change-Id: Ic5da8f2b469109cb10132cbe6585f2d941b14403
Reviewed-by: Andras Becsi <andras.becsi@digia.com>