[scan/library] Media rating sync (#1681)

Automatically read/write ratings to files in the library, if options read_rating/
write_rating are enabled. Also adds a max_rating so the user can set the rating
scale.

Doesn't sync automatic rating updates, because that could lead to whole-playlist
file rewriting.

Closes #1678 

---------

Co-authored-by: whatdoineed2do/Ray <whatdoineed2do@nospam.gmail.com>
Co-authored-by: ejurgensen <espenjurgensen@gmail.com>

owntone.conf.in

@@ -223,6 +223,16 @@ library {
 	# new rating = 0.75 * stable rating + 0.25 * rolling rating)
 #	rating_updates = false
 
+	# By default, ratings are only saved in the server's database. Enable
+	# the below to make the server also read ratings from file metadata and
+	# write on update (requires write access). To avoid excessive writing to
+	# the library, automatic rating updates are not written, even with the
+	# write_rating option enabled.
+#	read_rating = false
+#	write_rating = false
+	# The scale used when reading/writing ratings to files
+#	max_rating = 100
+
 	# Allows creating, deleting and modifying m3u playlists in the library directories.
 	# Only supported by the player web interface and some mpd clients
 	# Defaults to being disabled.