Use observers for callbacks

[janhohenheim]

• This variant is probably closer to what will be upstreamed in Bevy, as Alice mentioned wanting to solve this using observers.
• Its implementation is simpler (no need to learn about one-shot systems or SystemId, no need to be careful about memory leaks)
• I removed the extra param from button.
  • With the current code, the user does not save on boilerplate when passing the callback to the function. But it incurs a bit of mental indirection that is not present when observe is called directly and it requires us to add some ugly generics to the button function. I like about the current code that it kinda teaches the user how to apply observers to entities for themselves.
  • Of course, this means a user can forget to handle the OnPress event as a consequence.

[MiniaczQ]

This is way better than what was here previously!