Send current keyboard state when entering a surface

• Should we read the current keyboard state like this? The compositor might want to filter some keys, and we might end up in an inconsistent state if the compositor is simulating keypresses. Would it be better to take the state as a parameter to wlr_seat_keyboard_enter?
• Remove list of pressed keysyms in rootston, use list of pressed keycodes in wlr_keyboard? Can't fix easily after #394

Test plan: open two terminals. Press two times Ctrl + D without releasing Ctrl. This should close both terminals.

Fixes #389 (closed)