retrieve_widgets() earlier and use results initially when opening Customuzer

 * Run `retrieve_widgets()` earlier, and only when initially opening customizer in a theme switch.
 * Likewise, initially filter `sidebars_widgets` option to be the retrieved widgets.
 * Store the old theme's `sidebars_widgets` in a new customizer setting which gets supplied into the `sidebars_widgets` theme mod after theme activation.