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.Game/Map/CellCoordsRegion.cs

@@ -64,6 +64,11 @@ namespace OpenRA
 			BottomRight = bottomRight;
 		}
 
+		public bool Contains(CPos cell)
+		{
+			return cell.X >= TopLeft.X && cell.X <= BottomRight.X && cell.Y >= TopLeft.Y && cell.Y <= BottomRight.Y;
+		}
+
 		public override string ToString()
 		{
 			return $"{TopLeft}->{BottomRight}";