Introduce overridable wp.customize.Control.toggle() which handles active state change

Default behavior for widget with non-active state is to slideUp.
Interpret widgets not registered in the previewed URL as also being inactive.
Eliminate widget-specific message passing for rendered-widgets and rendered-sidebars

Use activeControls passed in ready message to determine widget/sidebar control active state.
Override wp.customize.Control.toggle for WidgetControl and SidebarControl.
Introduce WP_Customize_Widgets::is_widget_rendered().