Add event pubsub for glfw and jupyter canvas

[Korijn]

See pygfx/pygfx#223

New API implemented for glfw and jupyter canvases:

canvas.add_event_handler(callback, *types)
canvas.remove_event_handler(callback, *types)

@canvas.add_event_handler(*types)
def callback(event):
    ...

Where type can be any event from the spec and callbacks should be callables receiving an event object.