Improve nonlinear animation curve

[conan81412B]

These animation improvements will make the dash-to-dock feel more responsive than before.
#1620

[3v1n0]

I agree things are a bit snappier, just wondering if this makes things a bit less consistent.

Any thoughts @ewlsh, @micheleg?

[emansom]

How will it make things more responsive?

[conan81412B]

How will it make things more responsive?

I found a GIF from here. It can be seen that while the duration of the first two animations is the same, people feel that the second animation is more responsive (or faster). That's the difference between linear animation and non-linear animation - non-linear animation looks more "high-end".

In addition, it can also be noticed that more and more non-linear animations have appeared in recent GNOME versions (such as window maximization/minimization animations, activation animations of activity overview), it can be considered that GNOME has a tendency to replace linear animations with non-linear animations.

You can experience the different feel of different nonlinear animations here.

[nikodunk]

So this PR basically changes the animation from ease-out-quad to ease-in-out-cubic, which I believe will make the bar feel "stickier"?

ease-out-cubic could be another option, not implemented here. I wonder what macOS does?