Create a QmlEnginePrivate. Move the QmlDebugClient there.
Merge QmlAdapter into QmlEngine Abstraction is not used anymore.

Move some helper bits to a qmlengineutils.{h,cpp}

Change-Id: I63117355d786cc12641101b7fd38c7cd208d11eb
Reviewed-by: Christian Stenger <christian.stenger@theqtcompany.com>