keyboard: broadcast modifiers

keyboard: broadcast modifiers

...to all clients rather than just the one with keyboard focus on keyboard
enter/create, modifer press/release and wlr_seat_set_keyboard().

This enables:

- Clients such as panels to display the current keyboard layout without
  introducing new wayland protocols or other IPC.

- Unfocused xdg-shell clients to understand button press with keyboard
  modifiers for example Ctrl+click.

The keymap is forwarded to all clients in wlr_seat_set_keyboard(). When a
keymap contains multiple layouts, the selection is made via modifiers,
which previously were only sent to the client with keyboard focus.

Tested with: https://github.com/johanmalm/keyboard-layout

Ref: https://gitlab.freedesktop.org/wlroots/wlroots/-/merge_requests/4496

Fixes: #2271