Input: add keyboard notifier documentation

Keyboard notifier

One can use register_keyboard_notifier to get called back on keyboard

events (see kbd_keycode() function for details).  The passed structure is

keyboard_notifier_param:

- 'vc' always provide the VC for which the keyboard event applies;

- 'down' is 1 for a key press event, 0 for a key release;

- 'shift' is the current modifier state, mask bit indexes are KG_*;

- 'value' depends on the type of event.

- KBD_KEYCODE events are always sent before other events, value is the keycode.

- KBD_UNBOUND_KEYCODE events are sent if the keycode is not bound to a keysym.

  value is the keycode.

- KBD_UNICODE events are sent if the keycode -> keysym translation produced a

  unicode character. value is the unicode value.

- KBD_KEYSYM events are sent if the keycode -> keysym translation produced a

  non-unicode character. value is the keysym.

- KBD_POST_KEYSYM events are sent after the treatment of non-unicode keysyms.

  That permits one to inspect the resulting LEDs for instance.

For each kind of event but the last, the callback may return NOTIFY_STOP in

order to "eat" the event: the notify loop is stopped and the keyboard event is

dropped.

In a rough C snippet, we have:

kbd_keycode(keycode) {

	...

	params.value = keycode;

	if (notifier_call_chain(KBD_KEYCODE,&params) == NOTIFY_STOP)

	    || !bound) {

		notifier_call_chain(KBD_UNBOUND_KEYCODE,&params);

		return;

	}

	if (unicode) {

		param.value = unicode;

		if (notifier_call_chain(KBD_UNICODE,&params) == NOTIFY_STOP)

			return;

		emit unicode;

		return;

	}

	params.value = keysym;

	if (notifier_call_chain(KBD_KEYSYM,&params) == NOTIFY_STOP)

		return;

	apply keysym;

	notifier_call_chain(KBD_POST_KEYSYM,&params);

}

NOTE: This notifier is usually called from interrupt context.