Toggle steno input with the Sidewinder calculator button

My Sidewinder keyboard has this awesome little "Calculator" button in the upper right hand corner. With this patch, the calculator button becomes a first-class steno stroke, which means I can map it to `{PLOVER:TOGGLE}`, or any other key combination in `dict.json`. This is more convenient than mapping an actual steno stroke to `{PLOVER:TOGGLE}` because the calc button doesn't cause any on-screen input, so there's no need to press backspace to clean the gibberish up.

A better patch would add support for dedicated {f1}...{f12} keys or perhaps allow for modifiers {shift+calc}, etc.

This patch probably isn't a great way of doing this, but it scratches my itch, so I thought I'd share.