The protected methods of QmlDebugClient need to be called by
QmlDebugConnection already. We also want to call them directly for
testing, so we make them public rather than having QmlDebugConnection
be a friend. By using a QPointer, we can avoid resetting the connection
on each client when it is deleted.

Change-Id: I2c0e3d2b8ec19e9acbc6b8f5623b4c28caae319a
Reviewed-by: hjk <hjk@qt.io>