backends/wayland: Stop forwarding keymap and modifiers

These concepts are hard to map to InputDevice abstractions, so remove them and let kwin use its own internal keymap instead (rather than get it overridden by the host compositor's keymap) and track modifiers based on the events it receives.

If the goal is to have these at any cost, I think the way out of it would be to provide wayland backend specific signals. I would like to avoid doing that though because it breaks encapsulation and we would need some casts in kwin to wire in these events.