Customizer: Use correct event variable when moving widgets to another widget area. 

Fixes "ReferenceError: event is not defined error." in Firefox.

props tywayne.
fixes #30818.
Built from https://develop.svn.wordpress.org/trunk@30992


git-svn-id: http://core.svn.wordpress.org/trunk@30974 1a063a9b-81f0-0310-95a4-ce76da25c4cd