Add a simulacrum of C16+4 emulation.

[TomHarte]

Outstanding issues to be solved in the future:

• audio is missing noise generation and is probably wrong in its counting;
• there's no character mapper and, correspondingly, no pasting or automatic media starting;
• tape loading doesn't work (possibly an issue with the timers?);
• there's no GUI route to starting a machine with no media;
• not all TED registers are yet implemented — primarily around video counters; and
• the static analyser still routinely makes bad choices, especially with disk images.

Nevertheless I'm optimistic that this is reasonable as far as it goes, implementing a TED machine with decent timing and proper modelling of internal state, and I'm able to load from floppy disk.