Use QElapsedTimer for Compositor timer

Because it's the "Qt" thing to do.

Change-Id: Id85d1c3a51745d2c876b4337e90776628e79233a
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>
Reviewed-by: Jan Arne Petersen <jan.petersen@kdab.com>
Reviewed-by: Pier Luigi Fiorini <pierluigi.fiorini@gmail.com>