Improve the user’s control on the internal file storage

Right now, the Ace app uses a temporary directory as its internal file storage (for reports). This is configurable in the settings pane.

A couple ideas for improvement:
- [ ] set the default directory to a location in the user's data directory. That would be a nicer default, we just don't do it currently, as we don't provide an easy way to clean this directory (so there's a risk of bloating the users' space over time).
- [ ] add an option to delete the current report (and its associated data files)
- [ ] add a command to open the internal storage directory from the settings pane
- [ ] add an option to wipe-clean the internal storage directory (in the settings pane)