Add preliminary context menu support

This is essentially the widgets part, with some tricks to get it to
honor the widget's context menu policy.
It enables c++11 for the widgets library for the convenience of using lambdas,
which admitedly we could do without, but seems reasonable considering our timeline
and the fact that we build chromium that way.

Change-Id: I6a632a78d2aa48fb0dfecfe491e92651d12407db
Add javascript dialogs support

This is just the basic core part and widgets plumbing and default
implementations of the virtual functions in QWebEnginePage.

QtQuick implementation is still yet to be done

Change-Id: I7cf8d6e5ec0bf747d45e9914db57bd0e4ef95b7f
QtQuick Context menus

Change-Id: I853218b548ca0a6a01513a23cca8a4df300f51a6