unify i/o error reporting in the model

unify i/o error reporting in the model

- use Utils::FileSaver
- as a side effect, don't rewrite the desktop file with the same content
  if it already exists
- pop up message boxes on errors - this is likely to be more visible
  than qWarning()s/nothing
  - this is implemented in a somewhat strange way with the model itself
    doing the gui interaction, but that seems to be the simplest
    solution given the qt class design