Wayland: Keyboard input improvements (#7989)

Release Notes:

- N/A

---

Right now the Wayland backend is using `xkb::State::key_get_utf8` as the
`key`, when it should be used as the `ime_key`. It also manages
pressing/releasing modifiers manually when this should be managed by the
display server.

This allows modifier combinations to work in more cases, making it an
alternative to https://github.com/zed-industries/zed/pull/7975, which
interprets what is now only used as the `ime_key` value as a `key`
value.