wl_seat v10 with compositor key repeat

[dcz-self]

This adds compositor-side key repeat from the just released wl_keyboard v10.

There are some problems:

1. Repeat events have the same serial
2. Filtering may be broken
3. Arming the repeat timeout is ugly

With the above in mind, I'm not sure if I plugged it in the correct place, so I'm marking this as a draft.

[dcz-self]

I fixed the serial generation by pushing the logic up a bit, close to where the backend receives the event.

The events go now like:

1. comp: Backend
2. smithay: Repeat registration -> transforms backend event into generic event, adds Repeated
3. comp (via callback): event handler
4. comp: filtering -> adds Serial
5. comp: forwarding
6. smithay: normal client handling

This means the repeating is global to the seat: switch from one client to the other and repeating continues. If repeating was per-client, then the compositor itself would not receive repeat events without making it look like a client (and I'm not ready for that level of rearchitecting). The good news is I think feels more natural :) The bad news is corner cases when switching because a Repeated event may not appear without a Pressed event according to wl_keyboard:

The key may only enter the repeated state after entering the pressed
state and before entering the released state.

I haven't tried to add those corner cases yet.

EDIT: I read the spec more carefully and this is not needed.

Does this design look right?

[dcz-self]

This still relies on unreleased wayland-rs, but I copleted and undrafted it to reflect my confidence.