Checked LINQ queries and collections for inefficiencies.

- Made Array.IndexOf available via extension method.
- Made ToHashSet extension method.
- Change collections queried often via Contains into sets.
- Avoid Count() extension if Count or Length property exist.
- Made Count() > 0 checks and variations calls to Any() instead.
- Don't call ToList/ToArray if there is no benefit to materializing the sequence.
- If the sequence does benefit from materialization, follow this general pattern:
  - Collection queried often via Contains use ToHashSet to speed up lookups.
  - Short lived variables use ToList. This is because ToArray requires an extra copy to output the final size.
  - Collections persisted into fields or for a long time use ToArray to minimize memory overhead.

OpenRA.Editor/Form1.cs

@@ -170,7 +170,7 @@ namespace OpenRA.Editor
 			foreach (var p in palettes) { p.Visible = false; p.SuspendLayout(); }
 
 			var templateOrder = tileset.EditorTemplateOrder ?? new string[] { };
-			foreach (var tc in tileset.Templates.GroupBy(t => t.Value.Category).OrderBy(t => Array.IndexOf(templateOrder, t.Key)))
+			foreach (var tc in tileset.Templates.GroupBy(t => t.Value.Category).OrderBy(t => templateOrder.IndexOf(t.Key)))
 			{
 				var category = tc.Key ?? "(Uncategorized)";
 				var categoryHeader = new Label