Make QQmlDebugService::registerService() private and part of ctor

Make QQmlDebugService::registerService() private and part of ctor

By forcing all debug services to register before the thread starts we
can get rid of the complicated thread synchronization and have a more
natural API for the services.

We can also better enforce the thread situation when registering
services in QQmlDebugServer now. QQmlProfilerService should not
moveToThread() in its constructor as the thread has not been started,
yet. The thread affinity of QQmlProfilerService doesn't make any
difference anyway, as all relevant methods are protected by mutexes
and it doesn't have any slots.

Change-Id: I57db9e2bf94ec6884ede694715dadf5bfd687334
Reviewed-by: Simon Hausmann <simon.hausmann@theqtcompany.com>