Changed MiniYaml.NodesDict property into a method.

Method is now called ToDictionary.
- Cached a few invocations into locals which should prevent some redundant evaluation.
- Added ToDictionary overloads that take projection functions for the keys and elements, since several callsites were doing a subsequent Linq.ToDictionary call to get this.

OpenRA.Game/GameRules/MusicInfo.cs

@@ -24,7 +24,7 @@ namespace OpenRA.GameRules
 		{
 			Title = value.Value;
 
-			var nd = value.NodesDict;
+			var nd = value.ToDictionary();
 			var ext = nd.ContainsKey("Extension") ? nd["Extension"].Value : "aud";
 			Filename = (nd.ContainsKey("Filename") ? nd["Filename"].Value : key)+"."+ext;
 			if (!GlobalFileSystem.Exists(Filename))