QmlDebugging: Expand watch data that were previously expanded

The watch data model is cleared on every debug step. Hence,
tracking of expanded watch data is lost. Now, we keep track
of expanded watch data when stepping. However, we still lose
track of this data on a scope/stack change.

Task-number: QTCREATORBUG-8901
Change-Id: I1dc609cae1c3a36d8320864f71336b655eaa09d1
Reviewed-by: Kai Koehne <kai.koehne@digia.com>