Menu bars are containers that hold multiple menus. They change the behavior and appearence
of the md-menu directive to behave similar to an operating system provided menu.
You may place md-menu-items that function as controls within menu bars.
There are two modes that are exposed via the type attribute of the md-menu-item.
type="checkbox" will function as a boolean control for the ng-model attribute of the
md-menu-item. type="radio" will function like a radio button, setting the ngModel
to the string value of the value attribute. If you need non-string values, you can use
ng-value to provide an expression (this is similar to how angular's native input[type=radio] works.
Menus may be nested within menu bars. This is commonly called cascading menus.
To nest a menu place the nested menu inside the content of the md-menu-item.