QmlProfiler: Don't emit two changed() signals in a row

Without this all the data is processed another time in all timeline
models when the second signal is emitted. This is quite wasteful.

Task-number: QTCREATORBUG-10950
Change-Id: Iac1315689e0900abb48efc1cf6390ec559a1897c
Reviewed-by: Kai Koehne <kai.koehne@digia.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>