[kcmkwin/deco] Delay deleting of PreviewBridge

Our decoration is deleted using deleteLater() and that might cause access
to the bridge. Given that we also need to deleteLater() the
PreviewBridge.

To do so the PreviewBridge is no longer directly exposed to QML, but
in a wrapper object which holds the bridge as only element.

BUG: 344278
FIXED-IN: 5.4.3
REVIEW: 125724