Add UndoSystem implementation and refactor UndoNode for simplicity.

UndoNode.cc

@@ -1,15 +1,2 @@
+// Placeholder translation unit for UndoNode struct definition.
 #include "UndoNode.h"
-
-
-void
-UndoNode::DeleteNext() const
-{
-	const UndoNode *node = next_;
-	const UndoNode *next = nullptr;
-
-	while (node != nullptr) {
-		next = node->Next();
-		delete node;
-		node = next;
-	}
-}