Touch support implementation

[naithar]

Added initial touch input implementation.
Replaces #334 as it seems abandoned.
Required for #87 and probably #86
Related issue #300

Checklist

I confirm that I have done the following (if applicable):

• Added a changelog entry
• Added respective unit tests

[cart]

I'm thinking that we should make this the "go to" high level interface for touch interaction (much like the Input<T> abstractions). This would reduce the need to directly consume events.

Some thoughts on changes to make here:

• Rename TouchInputState, to the more friendly Touches
• Rename ActiveTouch to just Touch
• Add the "touch id" to ActiveTouch
• Add an iter() method to Touches, which iterates all Touches
• Make all Touches fields private + add relevant methods to expose functionality

That would result in user code looking something like this:

fn touch_system(touches: Res<Touches>) {
    for touch in touches.iter() {
        println!(
            "active touch: {} {} {} {}",
            touch.id, touch.position, touch.previous_position, touch.start_position
        );

        if touches.just_pressed(touch.id) {
            println!(
                "just pressed touch with id: {:?}, at: {:?}",
                touch.id, touch.position
            );
        }

        if touches.just_released(touch.id) {
            println!(
                "just released touch with id: {:?}, at: {:?}",
                touch.id, touch.position
            );
        }

        if touches.just_cancelled(touch.id) {
            println!("cancelled touch with id: {:?}", event.id);
        }
    }
}

[cart]

Partial example:

#[derive(Default)]
pub struct Touches {
    active_touches: HashMap<u64, Touch>,
    just_pressed: HashSet<u64>,
    just_released: HashSet<u64>,
    just_cancelled: HashSet<u64>,
}

impl Touches {
    pub fn iter(&self) -> impl Iterator<Item=&ActiveTouch> + '_ {
        self.active_touches.values()
    }

    pub fn just_pressed(&self, id: u64) -> bool {
        self.just_pressed.contains(&id)
    }

    pub fn iter_just_pressed(&self) -> impl Iterator<Item=&ActiveTouch> + '_ {
        self.just_pressed.iter().map(move |id| self.active_touches.get(id).unwrap())
    }
}

[cart]

Once we make "high level" touch changes, I think we should have two touch examples. One that illustrates how to use touch events (like the current example does) and one that illustrates the high level interface.

[cart]

lets use the bevy_utils::{HashMap, HashSet} here. They're still the std types, but they use the much faster aHash algorithm instead of the defaults.

[cart]

This looks good to me. I think we should make one of the two examples actually consume touch events, but I'll just do that in a followup pr so we can merge this faster.

Thanks!