Align and unify (most) modal and customizer icons. fixes #29147.

Built from https://develop.svn.wordpress.org/trunk@29542


git-svn-id: http://core.svn.wordpress.org/trunk@29318 1a063a9b-81f0-0310-95a4-ce76da25c4cd
TinyMCE wpView sandbox iframes: 
- Make them transparent.
- Don't load tons of unrelated styles that can break them. This could result in minimal styling mismatch to the front-end, but keeps the views working well and looking good.
Props avryl, fixes #29048.
Built from https://develop.svn.wordpress.org/trunk@29543


git-svn-id: http://core.svn.wordpress.org/trunk@29319 1a063a9b-81f0-0310-95a4-ce76da25c4cd
TinyMCE wpView: add sandbox iframe body styling to get proper height when resizing the iframe, fixes #29270.
Built from https://develop.svn.wordpress.org/trunk@29544


git-svn-id: http://core.svn.wordpress.org/trunk@29320 1a063a9b-81f0-0310-95a4-ce76da25c4cd
TinyMCE wpView: scale iframes to 100% width and ensure they have transparent background. See #29048.
Built from https://develop.svn.wordpress.org/trunk@29545


git-svn-id: http://core.svn.wordpress.org/trunk@29321 1a063a9b-81f0-0310-95a4-ce76da25c4cd
TinyMCE wpView: show an error when no items are found while parsing a audio/video/playlist shortcode. Props avryl, fixes #29114.
Built from https://develop.svn.wordpress.org/trunk@29546


git-svn-id: http://core.svn.wordpress.org/trunk@29322 1a063a9b-81f0-0310-95a4-ce76da25c4cd