Add stackable on_key_down and on_key_up event handlers

[Zoxc]

This adds stackable on_key_down and on_key_up event handlers.

[jrmoulton]

Should this be extended to all event handler?
I'm running into the same problem with on_focus on text input

floem/src/views/text_input.rs

Line 151 in 5b5b336

.on_event_stop(EventListener::FocusGained, move |_| {

[Zoxc]

Yeah, we probably want all view decorators to stack in general.