[internal] Separate packet enum from class

Pull the QRemoteObjectPacketTypeEnum definition out of the
QRemoteObjectPacket class.

This is in preparation for some performance changes.

Change-Id: Iedfa1798f18ec723327dc68a8ad039c6600a694a
Reviewed-by: Björn Breitmeyer <bjoern.breitmeyer@kdab.com>