Debugger: Add new debuggers from option page

The asserts should be moved from the addDebugger() method
which is called to add an 'empty' debugger when adding new one
from the option page.

Change-Id: Ic402db514d1e97f5fdc12d9bb9070bf29b2ab354
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: hjk <hjk121@nokiamail.com>