backend/wayland: fix seat caps handling
backend/wayland: fix seat caps handling

Previously, each time a wl_seat.capabilities event was received the
Wayland backend created new input devices. It now only does so the first
time.

Input devices are now destroyed when the cap is removed.

Closes: https://github.com/swaywm/sway/issues/5055

swaywm/wlroots,swaywm/wlroots,SirCmpwn/wlroots,ascent12/wlroots,ascent12/wlroots