Customizer: Trigger `widget-added` event when initializing.

Widget controls are now added to the pane dynamically via JavaScript, see #28709.
Remove the event trigger from `SidebarControl.addWidget()` as it's covered by `WidgetControl.ready()`.

props westonruter.
fixes #30236.
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git-svn-id: http://core.svn.wordpress.org/trunk@30307 1a063a9b-81f0-0310-95a4-ce76da25c4cd