Render the widgets view using the scene graph into a QOpenGLWidget

This means that widgets application now need to setup the GL context
sharing as well. QWebEngineWidgets::initialize() must be called,
which has the same effect as QWebEngine::initialize().
The QtWebEngineWidgets now depends on the QtWebEngine module to make
this happen.

Since QOpenGLWidget is only available in Qt 5.3, this patch also
disables the webenginewidgets module completely when building using
Qt 5.2.

Change-Id: I0e99a779d1eb080f2ccf5a338ff0763ad64e6eba
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>