''deck_setkey: <key> | <text> | <colour>''

Given that a Streamdeck is connected to the Wintermute PC, sets the given numbered key to the relevant text and colour.

  * **key** is the number of the key on the Streamdeck; if multiple Streamdecks are connected, they are treated as one large device.
  * **text** is the text to set the key to display.
  * **colour** is the colour to set the key to.

Wintermute is quite smart at keeping a dictionary of keys it has previously rendered.