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/ActorInfo.cs

@@ -33,7 +33,7 @@ namespace OpenRA
 		{
 			try
 			{
-				var mergedNode = MergeWithParent(node, allUnits).NodesDict;
+				var mergedNode = MergeWithParent(node, allUnits).ToDictionary();
 
 				Name = name;
 				foreach (var t in mergedNode)
@@ -49,7 +49,7 @@ namespace OpenRA
 		static MiniYaml GetParent( MiniYaml node, Dictionary<string, MiniYaml> allUnits )
 		{
 			MiniYaml inherits;
-			node.NodesDict.TryGetValue( "Inherits", out inherits );
+			node.ToDictionary().TryGetValue( "Inherits", out inherits );
 			if( inherits == null || string.IsNullOrEmpty( inherits.Value ) )
 				return null;