Ground work for splitting RenderWidgetHostViewQtDelegateQuick.

This prepares the addition of a delegated renderer implementation
for the QtQuick view.
Use a template for the base class since the accelerated delegate
will derive from QQuickItem instead of QQuickPaintedItem.

Change-Id: I5619290c0023bafdd27f18dab9edc10f5fbbf838
Reviewed-by: Zeno Albisser <zeno.albisser@digia.com>