QmlProfiler: Reduce code duplication between timeline models

Change-Id: Ic898ad06437209040c029304ee156f5aef5929da
Reviewed-by: Kai Koehne <kai.koehne@digia.com>
QmlProfiler: Unify source file lookup between V8 and QML models

They should both look up the source details when loading data
and they shouldn't do that again on gotoSourceLocation.

Change-Id: I433a4ff36a314822dff320ba4c8f398a6e3eaee3
Reviewed-by: Kai Koehne <kai.koehne@digia.com>