Some improvements for SpatiallyPartitioned.

- Tweak the Update and Remove methods to reduce the number of dictionary lookups required.
- Change the Update method to an indexer, this allows simplifying callers who wanted to AddOrUpdate a value.
- Implement IDictionary<T, Rectangle>, since the class already implements these semantics by providing a backing store of the bounds for each item.
- Clean up some naming to use the generic `item` instead of `actor`.
- Make the MutateBins action methods static.

OpenRA.Mods.Common/Traits/World/EditorActorLayer.cs

@@ -340,7 +340,7 @@ namespace OpenRA.Mods.Common.Traits
 
 		public void PopulateRadarSignatureCells(Actor self, List<(CPos Cell, Color Color)> destinationBuffer)
 		{
-			foreach (var preview in cellMap.Items)
+			foreach (var preview in cellMap.Keys)
 				foreach (var cell in Footprint(preview))
 					destinationBuffer.Add((cell, preview.RadarColor));
 		}