Addon-backgrounds: set background on iframe instead of adding a wrapper in preview

[Hypnosphi]

It fixes second part of #2655

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This change is 

[codecov]

Codecov Report

Merging #2807 into master will decrease coverage by 1.56%.
The diff coverage is 81.81%.

@@            Coverage Diff            @@
##           master   #2807      +/-   ##
=========================================
- Coverage   37.37%   35.8%   -1.57%     
=========================================
  Files         426     428       +2     
  Lines        9151    9422     +271     
  Branches      870     965      +95     
=========================================
- Hits         3420    3374      -46     
- Misses       5184    5410     +226     
- Partials      547     638      +91

Impacted Files	Coverage Δ
addons/background/src/index.js	80% <100%> (-6.37%)	⬇️
addons/background/src/BackgroundPanel.js	92.3% <80%> (-1.64%)	⬇️
app/vue/src/server/config/babel.js	0% <0%> (-100%)	⬇️
app/vue/src/server/utils.js	0% <0%> (-100%)	⬇️
app/react/src/server/babel_config.js	0% <0%> (-76.67%)	⬇️
addons/knobs/src/components/types/Select.js	7.93% <0%> (-16.71%)	⬇️
addons/knobs/src/base.js	6.66% <0%> (-5.1%)	⬇️
addons/info/src/components/types/OneOfType.js	57.14% <0%> (-2.86%)	⬇️
addons/info/src/components/types/PrettyPropType.js	85.71% <0%> (-0.29%)	⬇️
lib/ui/src/modules/ui/components/layout/index.js	29.28% <0%> (ø)	⬆️
... and 114 more

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[ndelangen]

@Hypnosphi We can actually improve this addon a lot.

We can set the iframe background to be transparent, and set the background in the manager.

What do you think?

[Hypnosphi]

@ndelangen Sorry, missed your comment.

Yeah, that sounds like a good idea, will try.

[Hypnosphi]

@ndelangen wouls your idea mean an API change? Currently the addon is used as decorator, which implies that it adds something to preview not manager

UPD: nevermind, I can just render the story as is

[Hypnosphi]

Another problem is, addons can't currently add things anywhere in manager outside of the addons panel. Should we allow them?

[ndelangen]

@Hypnosphi the viewport addon seems to be able to just fine?

[igor-dv]

LGTM, but I wonder if these changes are considered backward compatible. For example for people that use both, storyshots and background addons.

[Hypnosphi]

Do you mean that people will need to update their snapshots? To me personally it seems OK. Plus we're actually removing noise from storyshots: https://github.com/storybooks/storybook/pull/2807/files#diff-261e00eb9848ef08cfb37eb5e7e0b012

[igor-dv]

Yeah, that what I meant. Maybe it worth adding some note about it, Just to reduce confusion... I don't know where though

[Hypnosphi]

I'm pretty sure we're introducing changes that can affect existing snapshots all the time