Screenshots/soundlogs/taperecordings default location on Mac

On OS X, a directory that starts with a dot (e.g.
.openMSX) isn't shown in the Finder application, which
makes it very hard for users to find their
screenshots/soundlogs/taperecordings on OS X.

I think we should rename the user dir on OS X, or
define another directory where these output files end
up, which is easier to find with Finder.