Cover Image: Extract title as children array (fix "modified externally")

[aduth]

Related: #1929

This pull request seeks to resolve an issue where the Cover Image block can trigger a "modified externally" warning after adding inline formatting to the title text. This is caused by an incompatibility between how the attribute is sourced, and how it is assigned / edited. Specifically:

• The title is extracted as plain text
• But then edited as rich text

The value of Editable should always be an attribute which is sourced as children. We should consider adding better logging / messaging around the expected type of its value (i.e. warn if passed as string).

Testing instructions:

Verify with the following flow you do not see a "modified externally" warning:

1. Navigate to Gutenberg > New Post
2. Insert a Cover Image
3. Assign an image to the block
4. Add some text to the cover image
5. Select and apply inline formatting to text (Screenshot)
6. Save post
7. Refresh page

[codecov]

Codecov Report

Merging #2341 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #2341   +/-   ##
=======================================
  Coverage   24.69%   24.69%           
=======================================
  Files         152      152           
  Lines        4738     4738           
  Branches      799      799           
=======================================
  Hits         1170     1170           
  Misses       3014     3014           
  Partials      554      554

Impacted Files	Coverage Δ
blocks/library/cover-image/index.js	30.43% <100%> (ø)	⬆️

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