Debugger: Do not switch to undefined perspectives

This implies switching to Debug mode and is currently triggered
by the GammrayIntegration. Avoid it as long as GammaRay has
a mode of its own.

Change-Id: I6174554610038cb66fb1bc977dc537b47941a1d8
Reviewed-by: Christian Stenger <christian.stenger@theqtcompany.com>