Minor artwork caching changes

Art ID should be set on new files when they are stored into the
database, and the album art property should be affected by assigning to
the artId property, since it affects the caching identifier.

Signed-off-by: Christopher Snowhill <kode54@gmail.com>

Playlist/PlaylistEntry.m

@@ -120,7 +120,7 @@
 }
 
 + (NSSet *)keyPathsForValuesAffectingAlbumArt {
-	return [NSSet setWithObject:@"albumArtInternal"];
+	return [NSSet setWithObjects:@"albumArtInternal", @"artId", nil];
 }
 
 + (NSSet *)keyPathsForValuesAffectingGainCorrection {

Utils/SQLiteStore.m

@@ -1037,6 +1037,7 @@ static SQLiteStore *g_sharedStore = NULL;
 
 		if(albumArt)
 			artId = [self addArt:albumArt];
+		[track setArtId:artId];
 
 		st = stmt[stmt_add_track];
 
@@ -1232,6 +1233,7 @@ static SQLiteStore *g_sharedStore = NULL;
 
 		if(albumArt)
 			artId = [self addArt:albumArt];
+		[track setArtId:artId];
 
 		st = stmt[stmt_update_track];