Trigger some type of `ButtonEvent` on but input

[spvky]

What problem does this solve or what need does it fill?

This could just be a random pattern that's been on my mind, but with the inclusion of observer systems I've found they offer huge ergonomics win for a lot of different pieces of game logic and I wonder if it would be useful/desirable to have bevy trigger some sort of ButtonInputEvent<T> for things like keystrokes, mouse clicks, game pad button presses etc.

What solution would you like?

A simple, first class event that is triggered by bevy in response to user input

What alternative(s) have you considered?

This could fairly easily be done on an as need basis for anyone who desires this, I'm more gauging interest on if this would be worth adding?

p.s. I am not privy to any performance overhead that Trigger events that are not picked up have, so if It would be a disastrous feature to add that would be good to know as well

[spvky]

My initial thoughts on implementation:

1. Create a generic event type that accepts all of our first class input keys, most likely one for pressed and one for released
2. Wherever the existing logic is to mark a button as pressed/released, trigger a command for that same input event
3. Add an input example for consuming this pattern