Use widget set specified by init parameter (#20276)

Use widget set specified by init parameter (#20276)

1. If a UI has @Widgetset, use that
2. If a "widgetset" init parameter has been specified, use that
3. If there is an AppWidgetset, use that
4. Use DefaultWidgetSet

This changes the default value of the @VaadinServletConfiguration widgetset
parameter to "", to avoid always setting the "widgetset" init parameter when
@VaadinServletConfiguration is used.

Change-Id: I87f9aa294d31e62bb94b7e831dd7b25ca1ce483b