feat/3376 fix/2955 image alt

[Arukuen]

fixes #2955
fixes #3376

The fix for #2955 also considers the expected behavior from the previous issue #2524. That is:

• If uploaded media library image has existing alt text, should overwrite custom alt text
• If uploaded media library image has no alt text (empty or blank), should NOT overwrite custom alt text

For #3376, we should allow explicit empty string for alt since the Image Alt Text Box: links the W3 Alt standard (similar to native Image) which says empty string for alt (decorative) has different interpretation compared to undefined alt.

[github-actions]

🤖 Pull request artifacts

file	commit
pr3377-stackable-3377-merge.zip	a67e1f5

[github-actions]

Try this Pull Request in the WordPress playground: https://playground.wordpress.net/?mode=seamless#{"landingPage":"/wp-admin/post-new.php?post_type=page","preferredVersions":{"php":"latest","wp":"latest"},"steps":[{"step":"login","username":"admin","password":"password"},{"step":"installPlugin","pluginZipFile":{"resource":"url","url":"https://raw.githubusercontent.com/gambitph/Stackable/artifacts/pr3377-stackable-3377-merge.zip"}}]}

[bfintal]

Closed in favor of #3404 - added option to show empty alt tag