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/ModMetadata.cs

@@ -34,10 +34,11 @@ namespace OpenRA
 					continue;
 
 				var yaml = new MiniYaml(null, MiniYaml.FromFile(yamlPath));
-				if (!yaml.NodesDict.ContainsKey("Metadata"))
+				var nd = yaml.ToDictionary();
+				if (!nd.ContainsKey("Metadata"))
 					continue;
 
-				var mod = FieldLoader.Load<ModMetadata>(yaml.NodesDict["Metadata"]);
+				var mod = FieldLoader.Load<ModMetadata>(nd["Metadata"]);
 				mod.Id = m;
 
 				ret.Add(m, mod);