Add UndoSystem implementation and refactor UndoNode for simplicity.

UndoSystem.h

@@ -0,0 +1,45 @@
+#ifndef KTE_UNDOSYSTEM_H
+#define KTE_UNDOSYSTEM_H
+
+#include <string_view>
+#include "UndoTree.h"
+
+class Buffer;
+
+class UndoSystem {
+public:
+	explicit UndoSystem(Buffer &owner, UndoTree &tree);
+
+	void Begin(UndoType type);
+
+	void Append(char ch);
+
+	void Append(std::string_view text);
+
+	void commit();
+
+	void undo();
+
+	void redo();
+
+	void mark_saved();
+
+	void discard_pending();
+
+	void clear();
+
+private:
+	void apply(const UndoNode *node, int direction); // +1 redo, -1 undo
+	void free_node(UndoNode *node);
+
+	void free_branch(UndoNode *node); // frees redo siblings only
+	UndoNode *find_parent(UndoNode *from, UndoNode *target);
+
+	void update_dirty_flag();
+
+private:
+	Buffer &buf_;
+	UndoTree &tree_;
+};
+
+#endif // KTE_UNDOSYSTEM_H