[broadway] Emulate hardware keycodes in keyboard events

[broadway] Emulate hardware keycodes in keyboard events

We can't really know the client side keymaps, so we use the keysym
as the hardware keycode (essentially claiming to have a keyboard with
one key for all possible keysyms). This is not ideal, but its hard to
do better with no knowledge of the client side keyboard mappings.
(And html keyboard events suck badly...)