QmlDebugClient: Rename "status" to "state" for consistency

"state" is the more widely used term and we should follow that
convention. We also need to distinguish between the state of the
underlying network socket and the client itself. The change makes this
explicit.

As preparation for the upcoming centralized debug support the "State"
enum of the debug client is also moved into the QmlDebugClient class.

Change-Id: Ib9d7e03d23528f16ed696ed3518e813d11ea1c32
Reviewed-by: Kai Koehne <kai.koehne@digia.com>