KCompositeJob: port to new connect syntax. Improves performance.

Jobs are created quite often (e.g. in Akonadi's ItemSync)
so the performance of connect() itself showed up while profiling.
Old-style connect requires string comparisons...

This made ItemsyncTest::testFullSyncManyItems 1% faster, but
that's 3 million CPU cycles, and it makes job creation disappear
completely from the graph.