Add undo/redo infrastructure and buffer management additions.

UndoNode.cc

@@ -0,0 +1,15 @@
+#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;
+	}
+}