Support blueprint update animations in coordinator updates

[kyleve]

This PR updates the somewhat experimental ItemContentCoordinator APIs to properly support animations for Blueprint-based cells. It has a few changes:

1. Share the ViewAnimation type across both scrolling and item content updates, by renaming ScrollAnimation to ViewAnimation.
2. Remove the View/view from the ItemContentCoordinator – exposing this makes no sense for blueprint cells, and also, all updates to the item content should be done through apply updates, not through manually poking the view.
3. Update the machinery used to propagate animations from coordinators to actually properly wrap updates in an animation block.
4. Add a demo for showing how to do an "on tap" animation when a row is tapped.

[kylebshr]

I think this looks good other than a timer/race condition question

[kylebshr]

I don't really understand what these arrays are doing, what's the purpose?

[kyleve]

It's for tracking the number of calls to the func during testing, so I can make sure the number of calls is right. It could also just be a counter since the type here is Void, but I kept it like this since thats what the rest of the file does.

[kylebshr]

Does this timer need to be retained? I can never remember. Separately, does it need to be canceled if another update happened? e.g., what if update A happened with a 1s delay, then B with 0.3s, would be occur and then get reverted back to A?

[kyleve]

It doesn't need to be retained, it gets retained by the Runloop itself, and then fires and then is deallocated.

I didn't figure any queueing / collapsing of info was useful here, since it's a lot of extra behaviour, and might not be expected.

Good point about the disparate delays – I guess I should update the backing value immediately; OR alternatively, not pull the current value until the block executes (that actually seems better?)

[kyleve]

Going to go with the latter:

    public func update(
        animation: ViewAnimation = .default,
        after delay: TimeInterval = 0,
        update : @escaping (inout Item<Content>) -> ()
    ) {
        if delay > 0 {
            Timer.scheduledTimer(withTimeInterval: delay, repeats: false) { _ in
                var new = self.currentProvider()
                update(&new)
                
                self.updateCallback(new, animation)
            }
        } else {
            var new = self.currentProvider()
            update(&new)
            
            self.updateCallback(new, animation)
        }
    }

[kyleve]

Done