vod.ing v0.0.6

Desktop shell v0.0.6. Installed apps will offer this update on next launch.

• The download folder is now set and created automatically (Videos/vod.ing, Movies/vod.ing on macOS), including for existing installs, so "Open folder" always works. "Ask each time" is remembered.
• Clip downloads always get a file extension; the Save As… dialog no longer defaults to the bare clip id.
• Linux: the download-folder picker in settings now opens.