Twenty Seventeen: Add theme support for customize selective refresh widgets

[celloexpressions]

Literally every new theme needs to have this one line:

add_theme_support( 'customize_selective_refresh_widgets' );

Apparently that's missing from whatever base this theme was started from, so add it there right away also. Then, adjust as needed for any JS-based widget layouts like masonry (see Twenty Thirteen for an example). The UX improvement of this change is massive.

[karmatosed]

Discussion here: ​https://github.com/WordPress/twentyseventeen/issues/346 - moved over to trac after merge, so lets continue discussion here not on GitHub.

[celloexpressions]

Before

[celloexpressions]

With forthcoming patch.

[celloexpressions]

With 38400.diff​ the customizer preview no longer reloads for every new widget or widget reorder. It only reloads when the first widget is added to an area. This provides a much faster, more contextual experience for users.

Apologies for the size of the gifs, I picked bad widgets to demo this with quickly.

[davidakennedy]

In 38867:

Twenty Seventeen: Add theme support for customize selective refresh widgets

Props celloexpressions.

Fixes #38400.