Introduce HTML5 caption support.

When a theme supports HTML5 captions via add_theme_support( 'html5', 'caption' ), figure and figcaption will be used instead of div and p.

There's a bonus. But first, some history: Captions were introduced with an inline style set for the container. This remains, as it is there to force captions to wrap. But this inline style included an extra 10 pixels, which have vexxed theme developers for years. While these pixels were designed to ensure padding around floated images, modern themes handle this with grace. The additional pixels thus feel encumbering.

As the new HTML5 gallery support avoids outputting default gallery styles (again, irking theme developers for years; see #26697), the new HTML5 caption support will also ditch these 10 pixels of unwanted hand-holding. 

The 10 pixels are also removed entirely in the visual editor (and more styles may also disappear here; see #26642), giving themes the power necessary to match the frontend styles.

The filter img_caption_shortcode_width added in 3.7 to work around this madness (see #14380) is skipped entirely when the theme supports HTML5 captions.

props obenland, azaozz.
see #26642. also fixes #9066.

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