Convert File menu accelerators (hotkeys) to custom system (#38)

For the sake of cleanliness, this commit includes a few other changes.
The File menu is now a control array, and a number of File menu actions
have been moved to the software processor (to prevent copy/paste code in
the hotkey handler).

All File menu entries now have relevant hotkeys, handled completely
separate from VB's hotkey handler.

(This is a much bigger commit than I'd like at this stage in the beta
process, but oh well...)