Elaborate on JSDoc blocks for `media-editor.js`.

See #26870.


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Correct a type.

See #26870.


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Disambiguate `this` and use the `@this` annotation in `media-editor.js`. This will become way more necessary in other places, but a few examples here show how confusing it can be. 

See #26870.


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Remove debug cruft.

Props nacin.
See #26870.


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Make some `@param` types more specific in `media-models.js`.

See #26870.


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Eliminate the 1px padding adjustment on active buttons because it creates an un-clickable dead zone 1px high and spanning the entire width of the button. Modify the background color of active buttons for better contrast to make the active state more obvious.

See #26700, props johnbillion.


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TinyMCE: fix error trying to translate non-existent button "title" in the compat3x plugin. See #24067.
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Autosave: refactor autosave.js, use heartbeat for transport and move all "Add/Edit Post" related functionality to post.js. See #25272.

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