TinyMCE:
- Tighten up button styles, add :hover for .mce-active buttons.
- Pad only the first toolbar row and remove the padding in both toolbars if the editor doesn't have a DFW mode.
Part props avryl, see #27279
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[27640] supported a caption for audio and video shortcodes as part of a UX iteration for the related MCE views. These captions have since be excluded, so the extra code should be removed (it was there for this sole reason).

See #27320.


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Only set the `poster` attribute automatically for videos when the image does not match the default mime-type icon. `wp_prepare_attachment_for_js()` is used in multiple contexts. This context requires disambiguation.

See [27659].


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git-svn-id: http://core.svn.wordpress.org/trunk@27810 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Heartbeat: no longer "experimental", see #25272
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