WP_Widget: Introduce `is_preview()` method.

With the Widget Customizer it's possible that previewed widgets can leak data outside of Customizer, when the widget uses the cache API.
The Customizer calls the regular update callback which should already refresh the cache. Since cache additions aren't blocked yet the cache can be filled with preview data.
To prevent this issue `WP_Widget::is_preview()` will return true, when `$wp_customize->is_preview()` returns true. If `is_preview()` is true, cache additions are suspended via `wp_suspend_cache_addition()`. Make sure your object cache drop-in has implemented `wp_suspend_cache_addition()`.

`is_preview()` can/should also be used inside `WP_Widget::widget()`, see WP_Widget_Recent_Posts or WP_Widget_Recent_Comments for examples.

For more info see IRC logs: http://irclogs.wordpress.org/chanlog.php?channel=wordpress-dev&day=2014-04-02&sort=asc#m824279

props westonruter.
fixes #27538.
Built from https://develop.svn.wordpress.org/trunk@27966


git-svn-id: http://core.svn.wordpress.org/trunk@27796 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Fix whitespace around `$GLOBALS['_wp_deprecated_widgets_callbacks'] `.
Built from https://develop.svn.wordpress.org/trunk@27967


git-svn-id: http://core.svn.wordpress.org/trunk@27797 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Widget Customizer: Minor code cleanup.

* Remove type hinting from `WP_Customize_Widgets`
* Rename `inject_preview_css` to `print_preview_css`
* Remove stray `@todo`

fixes #27534.
Built from https://develop.svn.wordpress.org/trunk@27968


git-svn-id: http://core.svn.wordpress.org/trunk@27798 1a063a9b-81f0-0310-95a4-ce76da25c4cd