Theme Customizer: Preview the target theme during save to ensure all settings...

Theme Customizer: Preview the target theme during save to ensure all settings are properly registered. Temporarily revert to the active theme when running switch_theme. fixes #20508, see #19910.

Introduces WP_Customize->start_previewing_theme() and WP_Customize->stop_previewing_theme() to easily enable/disable the theme filters as needed.


git-svn-id: https://core.svn.wordpress.org/trunk@20584 1a063a9b-81f0-0310-95a4-ce76da25c4cd