Allow debug plugin registrations from other threads than main GUI

There's no real reason to disallow that. The only thing we want to check
there is that the registration doesn't take place in the debugger
thread. Disallowing registrations from anywhere but the main GUI thread
creates unnecessary problems with threaded QML profiling and debugging.

Change-Id: Ic72d19237e2ddba02cc88f7f5a0743f6640fed4d
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>