Add the ability to create migrations on a per-store level

Now stores store versions to the database itself.

osu.Game/Database/DatabaseBackedStore.cs

@@ -17,6 +17,8 @@ namespace osu.Game.Database
         protected readonly Storage Storage;
         protected readonly SQLiteConnection Connection;
 
+        protected virtual int StoreVersion => 1;
+
         protected DatabaseBackedStore(SQLiteConnection connection, Storage storage = null)
         {
             Storage = storage;
@@ -31,6 +33,28 @@ namespace osu.Game.Database
                 Logger.Error(e, $@"Failed to initialise the {GetType()}! Trying again with a clean database...");
                 Prepare(true);
             }
+
+            checkMigrations();
+        }
+
+        private void checkMigrations()
+        {
+            var storeName = GetType().Name;
+
+            var reportedVersion = Connection.Table<StoreVersion>().FirstOrDefault(s => s.StoreName == storeName) ?? new StoreVersion
+            {
+                StoreName = storeName,
+                Version = 0
+            };
+
+            if (reportedVersion.Version != StoreVersion)
+                PerformMigration(reportedVersion.Version, reportedVersion.Version = StoreVersion);
+
+            Connection.InsertOrReplace(reportedVersion);
+        }
+
+        protected virtual void PerformMigration(int currentVersion, int newVersion)
+        {
         }
 
         /// <summary>

osu.Game/Database/StoreVersion.cs

@@ -0,0 +1,15 @@
+// Copyright (c) 2007-2017 ppy Pty Ltd <contact@ppy.sh>.
+// Licensed under the MIT Licence - https://raw.githubusercontent.com/ppy/osu/master/LICENCE
+
+using SQLite.Net.Attributes;
+
+namespace osu.Game.Database
+{
+    public class StoreVersion
+    {
+        [PrimaryKey]
+        public string StoreName { get; set; }
+
+        public int Version { get; set; }
+    }
+}