Introduce file picker and GUI configuration with enhancements.

- Add visual file picker for GUI with toggle support.
- Introduce `GUIConfig` class for loading GUI settings from configuration file.
- Refactor window initialization to support dynamic sizing based on configuration.
- Add macOS-specific handling for fullscreen behavior.
- Improve header inclusion order and minor code cleanup.

UndoTree.h

@@ -2,7 +2,7 @@
 #define KTE_UNDOTREE_H
 
 #include "UndoNode.h"
-#include <memory>
+
 
 struct UndoTree {
 	UndoNode *root    = nullptr; // first edit ever