Customizer: Make panel title translatable too.

props michalzuber.
see #27406.
Built from https://develop.svn.wordpress.org/trunk@29135


git-svn-id: http://core.svn.wordpress.org/trunk@28919 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Media Grid improvements:

* Remove fixed elements.
* Adjusts toolbar and search, fixes #28833.
* Add link to Add New button, fixes #28825.
* Add "Edit Metadata" tab to all items, "Edit Image" only for images
* Make strings translatable.
* First pass for help tab.

see #28850.
Built from https://develop.svn.wordpress.org/trunk@29136


git-svn-id: http://core.svn.wordpress.org/trunk@28920 1a063a9b-81f0-0310-95a4-ce76da25c4cd