Customizer: Cannot Reorder/Move widget to new widget area in Firefox

[tywayne]

Steps to recreate:

1. Activate a theme with multiple sidebars registered (I was using TwentyFourteen).
2. Go to Appearance -> Customize, then select Widgets
3. Toggle view of widgets in one of the sidebars
4. Click Reorder, then the 'move widget' icon
5. This will toggle a list of other registered sidebar areas where you can move the widget to. Attempting to select one of them will do nothing but give ReferenceError: event is not defined error.

The error is in lines 684-693 of /wp-admin/js/customize-widgets.js

this.container.find( '.widget-area-select' ).on( 'click keypress', 'li', function( e ) {
    if ( event.type === 'keypress' && ( event.which !== 13 && event.which !== 32 ) ) {
	return;
    }
    e.preventDefault();
    selectSidebarItem( $( this ) );
} );

The parameter passed to the function is e but then referenced in the function as event

Chrome/Safari seem to ignore this and work just fine, but Firefox throws the error and doesn't allow the action.

[ocean90]

In 30992:

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.