linux/storagepath: Revamp handling of user-dirs
linux/storagepath: Revamp handling of user-dirs

- dont assume that ~/.config/user-dirs.dirs exists
- don't assume it does have contents or the relevant value
- don't use exception handling as happy path control flow
- when defaulting, use the path under HOME, not e.g. /Pictures

kivy/plyer,kivy/plyer,kivy/plyer