Debugger: Improve QModelIndex dumper

An unavailable display value doesn't mean an invalid index

Change-Id: I68baa09c9d6176dcd1a3f983ec375414292d7026
Reviewed-by: Eike Ziller <eike.ziller@digia.com>
Debugger: Sort thread model directly

Change-Id: I776687b6abdf45b86cf879d6c7d1164da37ac04f
Reviewed-by: hjk <hjk121@nokiamail.com>