Fix: Block movers don't appear on the playground and on the widgets page

[jorgefilipecosta]

The keyframe animations contained a comment saying they need to be duplicated, but they were not being duplicated in widgets and playground styles this made the block movers invisible.
This PR fixes the problem by duplicating the animation styles that needed to be duplicated.

How has this been tested?

I went to Gutenberg playground npm run playground:start. I added multiple blocks and verified the block movers appear when I hover a block.
I went to the Gutenberg widgets page /wp-admin/admin.php?page=gutenberg-widgets and repeated the test described above.

[youknowriad]

This makes me think this animations should probably be moved to the movers stylesheet, if it's common to all usage of the editor movers?

[jasmussen]

It looks like this animation is reused in a few places, for example for the transient state in the file block, and the saving state for the save indicator. I imagine we'll want to use this in additional places, so it's nice with a mixin to help us create consistent animations.

but it does feel weird to keep duplicating these keyframes.

As a fix to the problem, 👍 👍 from me, but CC: @kjellr if you have any ideas as to how we can better reuse this animation and avoid the duplication.

[youknowriad]

Is there a way to move this animation to the Animate component?

[kjellr]

Yeah, I was thinking it'd be great to consolidate all of the keyframe animations that we reuse into a single place — they're currently all over the place. The animate component might be a good place for that.

That definitely seems like something that could be handled in a separate PR though. This one seems 👍 to me.

[jorgefilipecosta]

Merging this as it looks like it got @kjellr approval, and with this changes on we can continue fixing bugs on the widget screen and make them easier testable. I will try to consolidate the style in the animate component as suggested in parallel.