AbstractThumbnailItem inherits from QQuickPaintedItem using QPainter to
do the fallback painting of icons.
The scene is adjusted to get the information from QQuickItem instead of
QDeclarativeItem. Clipping got a little bit more complex as the clip
path does not exist any more. To get it right the ThumbnailItem needs to
specify the parent it wants to be clipped to with the clipTo property.
E.g.:
clipTo: listView
The scene uses this clipTo parent item to correctly calculate the clip
region. Also the ThumbnailItem needs to have clipping enabled.
Note: this commit currently breaks TabBox as the qml and view are not
yet adjusted. In scripting the export of the item is disabled, but any
qml script using a ThumbnailItem would obviously also fail.
