Use pushState when preview URL changed

[westonruter]

See

wp-customizer-browser-history/customizer-browser-history.js

Lines 77 to 112 in a68c70b

	// @todo If newQueryParams.url !== oldQueryParams.url, do history.pushState?
	history.replaceState( newQueryParams, document.title, urlParser.href );
	}
	} );
	/**
	* Watch for changes to a construct's active and expanded states.
	*
	* @param {wp.customize.Panel|wp.customize.Section|wp.customize.Control} construct Construct.
	* @returns {void}
	*/
	component.watchExpandedChange = function watchExpandedChange( construct ) {
	if ( construct.active ) {
	construct.active.bind( component.replaceState );
	}
	if ( construct.expanded ) {
	construct.expanded.bind( component.replaceState );
	}
	};
	/**
	* Unwatch for changes to a construct's active and expanded states.
	*
	* @param {wp.customize.Panel|wp.customize.Section|wp.customize.Control} construct Construct.
	* @returns {void}
	*/
	component.unwatchExpandedChange = function watchExpandedChange( construct ) {
	if ( construct.active ) {
	construct.active.unbind( component.replaceState );
	}
	if ( construct.expanded ) {
	construct.expanded.unbind( component.replaceState );
	}
	};
	// @todo window.addEventListener( 'popstate', ... ) if pushState used.

On popstate we'd need to make sure that we only update the url and leave the autofocus params intact.

[westonruter]

This would be useful in Customize Posts if the preview is automatically navigated to when post sections expand.