Add observable collections

OpenRA.FileFormats/Primitives/IObservableCollection.cs

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+#region Copyright & License Information
+/*
+ * Copyright 2007-2013 The OpenRA Developers (see AUTHORS)
+ * This file is part of OpenRA, which is free software. It is made
+ * available to you under the terms of the GNU General Public License
+ * as published by the Free Software Foundation. For more information,
+ * see COPYING.
+ */
+#endregion
+
+using System;
+using System.Collections;
+
+namespace OpenRA.FileFormats.Primitives
+{
+	public interface IObservableCollection
+	{
+		event Action<object> OnAdd;
+		event Action<object> OnRemove;
+		event Action<int> OnRemoveAt;
+		event Action<object, object> OnSet;
+		event Action OnRefresh;
+		IEnumerable ObservedItems { get; }
+	}
+}