TODO

                       TO DO

* known bugs:
  - floppy: DOS 3.3 INIT command works, hangs, or
    returns I/O ERROR, depending on the disk
    (verification at VTRACK routine fails)
  - IN#0 in 80 col hangs
* keyboard:
  - nano timer to receive ESC key
* floppy disk support:
  - function key and menu item to hot-load a floppy
    into a drive
  - more disk formats (ProDOS, etc.)
* configuration:
  - find a way to control speed (for games)
    in addition to the `keyboard_poll_ratio` setting
  - allow to configure the retry delay
    for the Linux socket?
* Graphical User Interface:
  - blinking characters,
    including cursor in 80 col mode
  - high resolution graphics
* more hardware emulation:
  - game paddles
  - printer
  - serial card
  - Thunderclock Plus card
  - sound
* what about other Apple II models?