When reading a file:// URL from an xspf entry, store the decoded file path; else store the unmodified URL

Always keep album art URLs always encoded and decode the path when needed
Interfaces & plugins only use file paths
Playlist core extract attachment:// URLs (from file meta data), http:// or other (from meta data fetchers like lua) and cache them to a file

Thanks to courmisch for clarification
Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>
(cherry picked from commit a654d4a1)