QmlDebugging: Fix 'this' in Locals And Watchers

Incase of global object, query the children only when
needed as the response takes a lot of time. Remove cached
requests when switching stack frames so that stale responses
are not parsed.

Change-Id: I70887773952ecaa61da21077ffec321fd5fabbb1
Reviewed-by: Kai Koehne <kai.koehne@nokia.com>