Allow a single beatmap to reference the same file multiple times

This fixes incorrect reference counts causing database desync.

osu.Game/Beatmaps/BeatmapSetFileInfo.cs

@@ -2,16 +2,26 @@
 // Licensed under the MIT Licence - https://raw.githubusercontent.com/ppy/osu/master/LICENCE
 
 using osu.Game.IO;
+using SQLite.Net.Attributes;
 using SQLiteNetExtensions.Attributes;
 
 namespace osu.Game.Beatmaps
 {
     public class BeatmapSetFileInfo
     {
-        [ForeignKey(typeof(BeatmapSetInfo))]
+        [PrimaryKey, AutoIncrement]
+        public int ID { get; set; }
+
+        [ForeignKey(typeof(BeatmapSetInfo)), NotNull]
         public int BeatmapSetInfoID { get; set; }
 
-        [ForeignKey(typeof(FileInfo))]
+        [ForeignKey(typeof(FileInfo)), NotNull]
         public int FileInfoID { get; set; }
+
+        [OneToOne(CascadeOperations = CascadeOperation.CascadeRead)]
+        public FileInfo FileInfo { get; set; }
+
+        [NotNull]
+        public string Filename { get; set; }
     }
 }