Add basic realm models

Only the file related ones are really required outside of tests, but
seems like as good an opportunity as ever to get the rest of the models
into the game project.

osu.Game/Database/INamedFile.cs

@@ -0,0 +1,17 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+using osu.Game.Models;
+
+namespace osu.Game.Database
+{
+    /// <summary>
+    /// Represent a join model which gives a filename and scope to a <see cref="File"/>.
+    /// </summary>
+    public interface INamedFile
+    {
+        public string Filename { get; set; }
+
+        public RealmFile File { get; set; }
+    }
+}