Add undo/redo infrastructure and buffer management additions.

UndoTree.h

@@ -0,0 +1,20 @@
+#ifndef KTE_UNDOTREE_H
+#define KTE_UNDOTREE_H
+
+#include "UndoNode.h"
+
+class UndoTree {
+	UndoTree() : root{nullptr}, current{nullptr}, pending{nullptr} {}
+
+	void Begin(UndoKind kind, size_t row, size_t col);
+
+	void Commit();
+
+private:
+	UndoNode *root{nullptr};
+	UndoNode *current{nullptr};
+	UndoNode *pending{nullptr};
+};
+
+
+#endif // KTE_UNDOTREE_H