Model is extended to support QML based Aurorae themes as well
the preview is able to handle it. For this a new qml component
is added which loads the decoration preview.
Stop at bounds while flicking to have a list which behaves in a
natural way on the desktop and ensure that the scrollbar
handles cannot overshoot the list.
Thanks to Nuno for providing the QML based scrollbar. Obviously
this does not improve the consistency with other widget styles, so
a proper solution is still required.
BUG: 291612