Core color picker

[emeaguiar]

Summary

Fixes #354

Checklist

• My pull request is addressing an open issue (please create one otherwise).
• My code is tested and passes existing tests.
• My code follows the Contributing Guidelines (updates are often made to the guidelines, check it out periodically).

[coveralls]

Pull Request Test Coverage Report for Build 2865762665

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

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Details

• 8 of 8 (100.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.8%) to 72.97%

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Totals
Change from base Build 2835107180:	0.8%
Covered Lines:	4683
Relevant Lines:	6292

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💛 - Coveralls

[PatelUtkarsh]

Two things on this: @emeaguiar

1. useSetting is not available on 5.6 WP, which is our minimum support version.
2. Colour are not showing correctly on latest WP based on generated colors.

[PatelUtkarsh]

For point 2:

I cleaned up the existing global style in order for this to work:

DELETE FROM wp_posts WHERE post_type = 'wp_global_styles'