Use get_registered_nav_menus() to get a list of the theme's registered nav menu locations. Use this list to create a drop-down select for choosing the 'theme_location' option in the Navigation Menu widget.
Correct the 'classname' for all widgets, which somehow got reverted at some point.
Make sure the Calendar widget has a default title when added via the customizer or called via the_widget().
Add wrapper `<p>` for categories when there's no style.