Debugger: Add more safety net for failing dumper

In case a dumper explicitly reports the presence of children
but fails to extract them, remove the model's desire to
see them by re-setting the 'wantsChildren' hint explicitly
after a full update.

Change-Id: I09d3d3dd67067d22d39f7685b57570ecb42984d2
Reviewed-by: Christian Stenger <christian.stenger@qt.io>