Debugger: Introduce a "Native Mixed" switch

This paves the the way to mixed QML/C++ debugging
through the native backends. Currently this requires
QTC_DEBUGGER_NATIVE_MIXED to be set in the environment.

Change-Id: I126ad945e84806f3b548408318007351628c912f
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@theqtcompany.com>
Reviewed-by: Christian Stenger <christian.stenger@theqtcompany.com>