Redesign keyboard interface

This is a proposal to redesign the keyboard interface to be able to fix #242 (closed).

Right now, keyboard events follow this flow:

                                    +----> [ rootston ]
                                    |
[ backend ] +------> [ wlroots ] +--+
                                    |
                                    +----> [ client ]

So rootston has no control over keyboard events. It just selects the client to which events are sent to with wlr_seat_keyboard_notify_enter and wlr_seat_keyboard_clear_focus.

The idea is to change the flow to something similar to pointer events:

[ backend ] +---> [ wlroots ] +---> [ rootston ] +---> [ wlroots ] +---> [ client ]

With these functions:

• wlr_seat_keyboard_send_key, wlr_seat_keyboard_send_modifiers used by backends [same as right now]
• wlr_seat_keyboard_notify_enter and wlr_seat_keyboard_clear_focus used by rootston [same as right now]
• wlr_seat_keyboard_notify_key, wlr_seat_keyboard_notify_modifiers used by rootston [new]

Keyboard events won't be forwarded automatically by wlroots anymore, it'll be the compositor's role to listen to them and to call the two new functions.