Widget Customizer is incompatible with Twenty Fourteen Ephemera Widget

[celloexpressions]

Filing this as a widget customizer issue, since it should work with all existing widgets in theory. We could also fix this from the theme's side, though.

A few issues here. I'm guessing some might be known:

• Adding a Twenty Fourteen Ephemera widget through the widget customizer results in an error in the preview: Undefined index: format in wp-content\themes\twentyfourteen\inc\widgets.php on line 74
• Changing the format fixes the preview, but causes the widget controls to be replaced with: Notice: Undefined index: widget_twentyfourteen_ephemera-13 in wp-includes\class-wp-customize-widgets.php on line 1045
• I saw issues with MediaElement.js not being loaded when adding one with the post format set to audio or video at one point, although it doesn't appear to still be an issue.

[westonruter]

Ensure customizer is activated in update_widget ajax requests so that newly-added widgets will be recognized; fix positioning of Add Widget button when a widget area has many widgets in it (ensure it is always at the end)

[westonruter]

The issue was not limited to the Ephemera widget. I was able to reproduce the "Undefined index: widget_{widget_id}" issue by adding other widgets as well. The attached patch fixes the problem by ensuring that the update_widget Ajax request will boot up the customizer so that the necessary filters will be put in place to register newly-added widgets.

[westonruter]

Refreshed patch by resolving conflicts introduced by #27298. Commits (also rebased onto master) have also been pushed up to a branch on our GitHub clone of develop.git.wordpress.org: ​https://github.com/x-team/wordpress-develop/compare/trac-27291

[westonruter]

#27338 was marked as a duplicate.

[ocean90]

In 27524:

Widget Customizer: Use ?wp_customize=on.

Let _wp_customize_include() handle customizer initialisation.
Sets also the priority for the Add Widget and Reorder buttons and removes debug cruft.

props westonruter.
fixes #27291.