Debugger: Extract printing of dereferenced pointers into method

It's used twice, always for 'this', and whenever asked for by the
user's 'Dereference Pointers Automatically' choice, but needs to
be done at different times.

Change-Id: I4dc7340d0a4cb3eb0c7ff17d59e39be0e3870aa0
Reviewed-by: Christian Stenger <christian.stenger@qt.io>