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.

git-svn-id: https://develop.svn.wordpress.org/trunk@30308 602fd350-edb4-49c9-b593-d223f7449a82