Making so animation transition is able to handle multiple active animations!

[Sirmadeira]

Objective - Finished

In this PR I make it so AnimationTransitions component, actually has a concept called flows! Which will ease up the introduction of StateMachines for Animations
I also believe he shouldnt be limited by the idea of one sole "main" animation specially with the advent of layer through AnimationMasks. As the current implementation does

Solution

-In this case I make it so instead, of having the idea of one sole active animation, transitions actually has the ability of having multiple flows. Each flow having a one-to-one "main" animation. This will work nicely, with @pcwalton additive blends, and masks, and should mantain all the previous features of AnimationTransitions.

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[Sirmadeira]

@mweatherley @pcwalton Kindly review, would love this feature on 0.16.1.

[Lunkentuss]

Looks good overall and I like this change, and this is something I need myself. I have some comments however:

• I think it's unnecessary to introduce to much backwards incompatible changes for this. I would propose to keep the function signatures for the new and play as is, and introduce something like new_with_flow_amount and play_with_flow or something similarly. Not only would this be backwards compatible, but it would be a simpler API for consumers that doesn't care about flows - which I expect is and will be quite common.
• Remove ending exclamation marks (!) from the doc-strings you have changed/introduced, since this is a consistent with other documentation e.g. Animation Transitioning logic goes here!.

[Sirmadeira]

@Lunkentuss Makes sense

[Sirmadeira]

@alice-i-cecile If possible can we perhaps add this to the next merge train? I would love to take the glue code from my project?

[alice-i-cecile]

@alice-i-cecile If possible can we perhaps add this to the next merge train? I would love to take the glue code from my project?

This requires two approvals first; I'll ask around. I don't have the expertise with animation to review this myself. @Lunkentuss, if you're comfortable, we'd welcome a review from you however.