sends the modifiers on wlr_seat_set_keyboard

Created by: Ongy

Without this, a client will lose modifiers for one keyboard, when a key is pressed on the other. With this the client will always use the modifiers tate of the keyboard the key was pressed on.

Testplan: Connect 2 keyboard to the same seat, focus a window (I tested with weston-terminal). Hold shift on one keyboard and press a character key, this should input capital letters. Keep shift pressed, press a key on the other keyboard. Press the key on the first keyboard again. With this patch it should insert capital letters, without lower case.