Debugger: Introduce "Close memory buffers on exit" option

The usual case is probably to want normal editors stay open,
but memory/diassembler removed, but all other combinations
may be useful at times. So use two options instead of one.

Change-Id: I58c3951cc507c7e66a8d7a65b459a036e6a5d801
Reviewed-by: Christian Stenger <christian.stenger@theqtcompany.com>
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>