Improve performance of copy-paste in map editor.

- EditorActorLayer now tracks previews on map with a SpatiallyPartitioned instead of a Dictionary. This allows the copy-paste logic to call an efficient PreviewsInCellRegion method, instead of asking for previews cell-by-cell.
- EditorActorPreview subscribes to the CellEntryChanged methods on the map. Previously the preview was refreshed regardless of which cell changed. Now the preview only regenerates if the preview's footprint has been affected.

OpenRA.Mods.Common/Widgets/Logic/Editor/MapEditorSelectionLogic.cs

@@ -11,7 +11,6 @@
 
 using System;
 using System.Collections.Generic;
-using System.Linq;
 using OpenRA.Graphics;
 using OpenRA.Mods.Common.EditorBrushes;
 using OpenRA.Mods.Common.Traits;
@@ -123,7 +122,7 @@ namespace OpenRA.Mods.Common.Widgets.Logic
 				tiles.Add(cell, new ClipboardTile(mapTiles[cell], mapResources[cell], resourceLayer?.GetResource(cell), mapHeight[cell]));
 
 				if (copyFilters.HasFlag(MapCopyFilters.Actors))
-					foreach (var preview in selection.CellCoords.SelectMany(editorActorLayer.PreviewsAt).Distinct())
+					foreach (var preview in editorActorLayer.PreviewsInCellRegion(selection.CellCoords))
 						previews.TryAdd(preview.ID, preview);
 			}