Improve QBluetoothTransferManager et al API

1.) Analog to QNAM the QBluetoothTransferReply should have a reference to the associated QBluetoothTransferRequest

2.) QBluetoothTransferReply::attribute() is never set as the request values are not copied over or dynamically retrieved. Dynamic retrieval would imply point 1 above.

3.) The semi-private bluez subclass for BluetoothTransferReply (QBluetoothTransferReplyBluez) extends the QBluetoothTransferReply class with public functions (e.g. start()). At the very least those functions should be protected.

This changes API elements and must be done before the final release.