More feature-rich and better engineered.

* Instead of needing to press backspace, going back is now handled via an "(up arrow) UP..." entry in the table.
* Maximum depth is now #define'd instead of dotted around everywhere.
* Enums!
* Exec for .txt (with confirmation dialog) and .cfg (without)!
* Complaints about folders and etc now mention the folder names.
* A shrinking folder now prevents dir_on[] from going beyond its end.