This introduces quite some changes. We cannot include a QQuickView
directly in the QWidget based UI as a replacement for the
QDeclarativeView used before.
Instead a QScrollArea is used as replacement for the view and a
QQuickView is embedded into a widget container in the scroll area's
view port. This allows us to keep the existing semantic of having a
native scroll bar synced with the list view. It might be an idea to
change this to QtQuickControls in future.
As the list view is now only inside the scroll area we do not need to
pass the slider width to the root context and can by that more easily
calculate the width of one decoration.
