Allow multiple wl_keyboards, wl_pointer, etc. under single wl_seat

The current paradigm for input devices is that under one wl_seat all physical devices are grouped together to single wl_keyboard, wl_pointer, etc. instances.

This was meant as a simplification in comparison to the old X11 device handling but has led to some issues. For example having multiple keyboards connected with different keymaps connected under a single wl_seat is in principle not possible. One can connect such and the compositor updates the keymap in between, but this breaks if keys are pressed simultaneously.

With !62 there is an attempt to solve this through logical keyboard groups, but it would be better to directly allow, maybe if not currently possible in a Wayland 2.0 version, compositors to create multiple wl_keyboards, etc. under a single seat and let clients handle the multiplexing.