Create cover block with only colour

[antonis]

Fixes WordPress/gutenberg#23831 (parent #2275)

Related PRs:

• Gutenberg [Mobile] Create cover block with only colour (from a default palette) without an image WordPress/gutenberg#23833
• Clear media button in block settings [Mobile] Cover: Adds Clear media button in block settings WordPress/gutenberg#23794
• Create cover block with custom colour [Mobile] Create cover block with custom colour (without an image) WordPress/gutenberg#23870
• Media Button when only colour is selected [Mobile] Cover: Add Media Button when only colour is selected WordPress/gutenberg#23878

Description

A palette with four colours (black, white and a couple of theme defaults) has been added bellow the Add Media Button. When the user taps on a colour indicator the block is created with the selected colour as background.
The latest design can be found here.

How has this been tested?

• Add a cover block
• Press the one colour from the palette
• Verify that a new cover block with only colour is created

Screenshots

Empty cover block	Android	iOS

Types of changes

Enhancement

PR submission checklist:

• I have considered adding unit tests where possible.
• I have considered if this change warrants user-facing release notes more info and have added them to [RELEASE-NOTES.txt](RELEASE-NOTES.txt) if necessary.

[peril-wordpress-mobile]

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[geriux]

Approved via WordPress/gutenberg#23833 (review)