Adds the support for saving playlists with relative paths in cases where the music files are located in the same folder as the playlist to be saved. This setting is "Prefer relative paths" by default as that seems to be common with most music players.
A new persistent config
alwaysUseAbsolutePlaylistPaths is created, which is then checked in
MediaPlayListProxyModel::savePlaylist. With a false value, music files in the same folder as the playlist are pathed with just the filename, otherwise with absolute paths. With a true value an absolute path is always used.