Add TweenAnim<T>

[1ADIS1]

The ability to define animations that are running in parallel in a single line was a really powerful concept. Previously, we had Tracks feature for this. And when Animator<T> was abstract over some component, it also allowed to create several animator components on a single entity. Also, I think TweenAnim<T> explicitly defined, which component is being animated instead of relying on separate component AnimTarget::component::AnyComponent. So, if we will go with TweenAnim<T>, we should remove AnimTarget.

Right now , you cannot create several TweenAnim on the same entity, and you are required to create another entity for each animation, which creates a lot of boilerplate code.

I propose to introduce TweenAnim<T> to allow for creating several animations on 1 entity. To make animations in parallel you will just have to create TweenAnim<T>, TweenAnim<C>, and etc. on the same entity, which is really simple and easy way to animate entities.

[djeedai]

You're basically asking to revert the entire change of last version. This is not something I'll consider, given how many users explicitly wanted that change and an explicit animation target. Having the component as a type is an awkward middle ground where you can add multiple animations on the same entity for different components but not for the same one. This was very confusing to most users. It's also a lot more difficult to optimize, and requires one system per component. Those are all drawbacks the last refactor solved.