detect physical keyboards

see phosh#206 (comment 69558) on what we currently handle as such.

We could filter these out via libinput_device_keyboard_has_key in handle_new_input but these still send interesting button events so i don't think we can not announce them to clients as keyboards - we likely need some protocol to announce 'full physical keyboard availability' on a seat or am i missing something and it's possible with what we already have?