Allow saving in the background

When saving a score, either manually or automatically (autosave), MuseScore freezes until the saving process is complete. In the case of a large score this may take several seconds, during which no action can be done (except going on with playback if it started before saving).

It would be important that the UI kept accepting commands from the user during the saving process, even if their effect were delayed. In particular, the possibility of navigating the menus is crucial.