introduce module specific cancels

[red031000]

instead of doing !cancel to cancel the currently running command or !stop to stop all commands, consider adding a !# cancel command to stop the next command for that module or a !# stop command to stop all commands for that module

[Timwi]

This is a really good idea. I thought we already had this and I was just always too slow to cancel in time.

When you implement this, make sure you don't cancel an ongoing command while a button is being held. Many modules with long-press capabilities (e.g. Black Hole, The Bulb, Tennis) assume that buttons aren't held between commands and would break unrecoverably if that were to happen.