Register as handler for audapolis files

We could register as a handler for `.audapolis`-files so audapolis is automatically opened for such files and users can drag audapolis files onto the taskbar/dock-icon.

This would require us to have a mechanism to keep a list of to-open files in the main process (also see the discussion #302)

See https://github.com/rchrd2/example-electron-file-association for an example on how to implement parts of this feature