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.Game/Widgets/WidgetUtils.cs

@@ -75,7 +75,7 @@ namespace OpenRA.Widgets
 		public static int[] GetBorderSizes(string collection)
 		{
 			var images = new[] { "border-t", "border-b", "border-l", "border-r" };
-			var ss = images.Select(i => ChromeProvider.GetImage(collection, i)).ToArray();
+			var ss = images.Select(i => ChromeProvider.GetImage(collection, i)).ToList();
 			return new[] { (int)ss[0].Size.Y, (int)ss[1].Size.Y, (int)ss[2].Size.X, (int)ss[3].Size.X };
 		}