Re-read FMPS_Rating from file when changed

When the FMPS_Rating changes in a file, Clementine will now re-read it
and change the rating in the library database.  This lets you modify
song ratings outside of Clementine and then load the new ratings into
Clementine.  Previously Clementine would ignore changed ratings in a
file's metadata after a song had been scanned into the library.

src/core/song.cpp

@@ -1065,6 +1065,7 @@ bool Song::IsMetadataEqual(const Song& other) const {
          d->samplerate_ == other.d->samplerate_ &&
          d->art_automatic_ == other.d->art_automatic_ &&
          d->art_manual_ == other.d->art_manual_ &&
+         d->rating_ == other.d->rating_ &&
          d->cue_path_ == other.d->cue_path_ &&
          d->lyrics_ == other.d->lyrics_;
 }
@@ -1141,7 +1142,6 @@ void Song::MergeUserSetData(const Song& other) {
   set_playcount(other.playcount());
   set_skipcount(other.skipcount());
   set_lastplayed(other.lastplayed());
-  set_rating(other.rating());
   set_score(other.score());
   set_art_manual(other.art_manual());
 }