QmlDebugClient: rename serviceVersion to remoteVersion and return int

The name change is in preparation for the upcoming unification of the
debug server and client APIs. Expressing versions as floats is pretty
useless as we cannot properly compare them. Besides, so far we're using
exactly 3 different versions: -1 (a.k.a. invalid), 1, and 2.

Change-Id: Ie4012d09362009b08a1263b15bc552731a1f255e
Reviewed-by: Kai Koehne <kai.koehne@digia.com>