Add undo/redo infrastructure and buffer management additions.

GUIInputHandler.h

@@ -27,6 +27,8 @@ private:
 	std::mutex mu_;
 	std::queue<MappedInput> q_;
 	bool k_prefix_ = false;
+	// Treat ESC as a Meta prefix: next key is looked up via ESC keymap
+	bool esc_meta_ = false;
 	// When a printable keydown generated a non-text command, suppress the very next SDL_TEXTINPUT
 	// event produced by SDL for the same keystroke to avoid inserting stray characters.
 	bool suppress_text_input_once_ = false;