Remove event handlers to CellEntryChanged when done.

Several classes would attach event handlers to the Map which would live longer then they did. Detaching them when no longer needed allows the GC to reclaim them.

OpenRA.Mods.Common/Widgets/RadarWidget.cs

@@ -442,5 +442,12 @@ namespace OpenRA.Mods.Common.Widgets
 			var v = (int)((p.Y - mapRect.Y) / previewScale) + world.Map.Bounds.Top;
 			return new MPos(u, v).ToCPos(world.Map);
 		}
+
+		public override void Removed()
+		{
+			base.Removed();
+			world.Map.MapTiles.Value.CellEntryChanged -= UpdateTerrainCell;
+			world.Map.CustomTerrain.CellEntryChanged -= UpdateTerrainCell;
+		}
 	}
 }