Optimize Song::InitFromQuery

Use `QSqlQuery::value(int)` or `QSqlRecord::value(int)` instead of `QSqlQuery::value(QString)`.
Make `SqlRow` use `QSqlRecord` directly instead iterating over all columns.

src/playlist/songplaylistitem.cpp

@@ -33,7 +33,7 @@ SongPlaylistItem::SongPlaylistItem(const Song::Source source) : PlaylistItem(sou
 SongPlaylistItem::SongPlaylistItem(const Song &song) : PlaylistItem(song.source()), song_(song) {}
 
 bool SongPlaylistItem::InitFromQuery(const SqlRow &query) {
-  song_.InitFromQuery(query, false);
+  song_.InitFromQuery(query, false, Song::kRowIdColumns.count() * 3);
   return true;
 }