Add native support for tabbed settings pages in WP Admin

[stevegrunwell]

There are a lot of plugins out there handling tabbed setting screens in different ways, so it may be time to introduce a standard API for this pattern.

This patch adds a new do_tabbed_settings_sections() function which works just like the Settings API's do_settings_sections() function, but prints each section heading as a tab/panel combination. In situations where there's only one section to render, the new function will fall back to its un-tabbed sibling.

The patch also includes a standard script for tabbing between different setting sections, with support for linking directly to a particular tab via the URL hash (e.g. https://example.com/wp-admin/options-general.php?page=some-settings-page#some-tab-ID).

[stevegrunwell]

Introduces do_tabbed_settings_sections()

[stevegrunwell]

A demo plugin that registers both a tabbed and non-tabbed settings page for testing/comparision

[stevegrunwell]

Removes short-array syntax in test fixture

[stevegrunwell]

Fresh patch that actually includes the JavaScript

[stevegrunwell]

Remove reliance on JavaScript globals, use printf() for previously-concatenated string (props @szepeviktor)

[stevegrunwell]

With some assistance from @szepeviktor, I've open-sourced a polyfill for plugin authors looking to start using do_tabbed_settings_sections(): ​https://github.com/stevegrunwell/wp-admin-tabbed-settings-pages

[stevegrunwell]

Prevent <section> elements from being broken if no fields are registered for a setting (see ​https://github.com/stevegrunwell/wp-admin-tabbed-settings-pages/pull/11)

[stevegrunwell]

Refreshed diff, corresponding to v0.2.0 of the polyfill: ​https://github.com/stevegrunwell/wp-admin-tabbed-settings-pages/releases/tag/v0.2.0

[joedolson]

Maybe 6.1. Will try and keep track of this; depends on the upcoming release schedules.