Change to/fromAscii to to/fromUtf8 in the QString conversions

Change to/fromAscii to to/fromUtf8 in the QString conversions

This commit matches the previous documentation commit that says that
the conversions are applied using to/fromUtf8.

Change-Id: I304e4d866ddedac5094fef8500cbeba299a02cb5
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>
Reviewed-by: hjk <qthjk@ovi.com>
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Reviewed-by: Lars Knoll <lars.knoll@nokia.com>