Debugger: Make DebuggerEngine internal

Expose less of debugger internals to the device plugins.

Change-Id: Ie78756b033fae3fe02bff92eadf26b7319990605
Reviewed-by: Christian Stenger <christian.stenger@theqtcompany.com>
Reviewed-by: Christian Kandeler <christian.kandeler@theqtcompany.com>
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>
Reviewed-by: hjk <hjk121@nokiamail.com>