QDBusServiceWatcher: Move the logic to QDBusConnectionPrivate

With kdbus, we won't have a regular signal, but instead a special
message. So keep the logic of what to do in QDBusConnectionPrivate.

The #ifdef is to make sure the bootstrapped qdbuscpp2xml continues to
build in cross-compilation environments.

Change-Id: Iee8cbc07c4434ce9b560ffff13d06f0d9904cb6d
Reviewed-by: Alex Blasche <alexander.blasche@theqtcompany.com>