Media grid: Don't show a misleading message for an empty library.

This uses the existing string from the list table. We should revisit a better overall experience for the two situations (empty library vs. search with no results) in 4.1.

props voldemortensen.
fixes #29404.

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Editor expand:
- Adjust calculation when comparing sidebar position to wpfooter top. Fixes an annoying jumping when the user scrolls off the bottom.
- Use a var for the 'bottom' distance of the sidebar.
- Fix sidebar pinning when it's height is nearly the same as the viewport height.
Fixes #29349
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TinyMCE: fix the 'tabfocus' plugin to check if default is prevented on pressing tab, bind the events later so other plugins can cancel them. Merged upstream in https://github.com/tinymce/tinymce/commit/86098b348582e1544cc208db25954d31755e6d13 together with another change.
Fixes #29430
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Query: Add comment to the parse_tax_query() method reflecting [28987].

see #28739.

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Media: Translate a string for the search placeholder.

props alex-ye.
fixes #29448.

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git-svn-id: http://core.svn.wordpress.org/trunk@29433 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Media: add a class to the <script> tag for the JSON encoded playlist data so it can be easily selected in WPPlaylistView. Props SergeyBiryukov, fixes #29383
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