Coding standards and readability spacing for WP_Customize_Widgets.

Also explicitly re-mark `_capture_filter_pre_update_option()` and `_capture_filter_pre_get_option()` as public.

See #27534.


git-svn-id: https://develop.svn.wordpress.org/trunk@27904 602fd350-edb4-49c9-b593-d223f7449a82