wayland: use new function xkb_keymap_from_buffer

Bump xkbcommon version and use the new xkb_keymap_from_buffer. This is more
secure, because the from_string expects a 0 terminated string, but this cannot
be guaranteed with mmap.
wayland: remove unused stuff
wayland: avoid iterating over all outputs

If we use the output itself as user data we don't have to iterate over all
outputs when the handle_mode event comes.