Use system directory chooser to select location for automatic backups #1064

The current DIY filepicker has some limitations, biggest of which is that you can't add existing locations. I keep my Backups in Internal/Backups/[App], which is synced via syncthing. This works quite well for everything except the most important stuff (contacts, calendars, notes).

Would it be possible to use the system file picker for choosing the location instead?