Add a "Create New Menu" button to the menu widget

[celloexpressions]

Menu widgets in the customizer have buttons to edit the selected menu, which cross-link to the menus customizer panel. Before a menu is selected, there should be a "Create New Menu" button that links to the new menu section and automatically assigns the menu to the corresponding widget.

This would be a big improvement in user flow, particularly for new users trying the menu widget for the first time, who may not understand the relationship between widget and menu objects. The UI should match the existing edit menu button.