wip: keyboard repetition

[wash2]

This is a refactor of https://github.com/Smithay/client-toolkit/pull/234 . It uses the latest Calloop Timer, and I adapted the simple window to include its usage.

The only point that I am unsure about handling timer stop & start. It wants to remove itself from the event loop after TimeoutAction::Drop, or it disagrees with the channel post action, and they continually re-register, which seems to cause busy looping. I hope I've handled it well, as my current implementation here seems to avoid the repeated calls to processing,