dvr_storage: Use XDG spec directories

dvr_storage: Use XDG spec directories

The XDG spec has a way to determine the users preferred Video directory.
This is important, because in different locales, this may be a different
directory, preventing annoyance for users who have a localized home dir.

With the newly added XDG helpers, this becomes a triviality.

This change does mean, that the behavior is slightly changed, as XDG
directories are probed first.

However since this only affects the startup, after which these
directories are stored in the config, the impact should be neglectable.

Signed-off-by: Olliver Schinagl <oliver@schinagl.nl>