Media Grid: add Bulk Selection mode for deleting attachments.

* Toolbar is sticky when `select` mode is active
* Selection is toggled when clicking an attachment preview
* Unselected attachments fade out, selected fade in.

See #28842.

Built from https://develop.svn.wordpress.org/trunk@29484


git-svn-id: http://core.svn.wordpress.org/trunk@29262 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Remove debug line from [29484].

See #28842.


Built from https://develop.svn.wordpress.org/trunk@29485


git-svn-id: http://core.svn.wordpress.org/trunk@29263 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Remove now-unused strings. see #28842.

Built from https://develop.svn.wordpress.org/trunk@29486


git-svn-id: http://core.svn.wordpress.org/trunk@29264 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Separate WP_Customize_Panel from WP_Customize_Section.

props celloexpressions.
fixes #29197.

Built from https://develop.svn.wordpress.org/trunk@29487


git-svn-id: http://core.svn.wordpress.org/trunk@29265 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Customizer: Mixed priority sorting for panels and top-level sections.

props celloexpressions.
fixes #28979.

Built from https://develop.svn.wordpress.org/trunk@29488


git-svn-id: http://core.svn.wordpress.org/trunk@29266 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Media grid bulk selection styling:

* Fade content within each attachment item so that focus styling is still clear.
* Less jarring toolbar pinning.
* Tighten up the fade in/out transition.

see #28842.

Built from https://develop.svn.wordpress.org/trunk@29489


git-svn-id: http://core.svn.wordpress.org/trunk@29267 1a063a9b-81f0-0310-95a4-ce76da25c4cd