seat: only pass on sent keys on surface-focus

seat: only pass on sent keys on surface-focus

Key events associated with keybindings (both pressed and released) are not
sent to clients. When using wlr_seat_keyboard_notify_enter() it it
therefore important not to send the keycodes of _all_ pressed keys, but
only those that were actually _sent_ to clients (that is, those that were
not bound).

This approach is consistent with sway's implementation in input/seat.c
https://github.com/swaywm/sway/blob/cffb006feba52c318e66f73c3463032fa76782dc/sway/input/seat.c#L173-L175

Fixes issue #510