Added keystrokes to the input

[VictorKoenders]

When trying to make a text input component, I found out there's no way to detect what keys were pressed this tick.

The current implementation is lazy, although we might want to make this allocated since the returned iterator borrows the given Context. On the other hand, it's fairly easy to call .collect::<Vec<_>>()

I didn't think a get_key_releases method would be useful, but I'll add it if you think otherwise.

[17cupsofcoffee]

This is excellent, had been meaning to get around to adding something like this :)

Couple of comments:

• 🚲🏠: Could we call this method get_keys_pressed? I feel like that's a bit more consistent with the other methods, and I might add get_keys_down, get_keys_released, etc later on.
• I think you might be able to simplify the implementation to ctx.input.current_key_state.difference(&ctx.input.previous_key_state). This also has the nice side effects or removing the cloning, and removing the need for the explicit lifetimes on the method.

[VictorKoenders]

On second thought, the correct way to handle text is through the Event::TextInput.

Let me close this and try something with that.

[VictorKoenders]

Let me know if you still want this PR, but I think the TextInput event supersedes this.

[17cupsofcoffee]

I can think of a couple of places where this would come in handy outside of text input - that said, I'll make the above changes myself rather than making you do them, if the text input stuff is what you're mainly interested in :)