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
cffb006feb/sway/input/seat.c (L173-L175)
Fixes issue #510