Move "Expanded" option for Menu Items to Block settings

Description of the need

Follow-up to https://github.com/backdrop/backdrop-issues/issues/2782 and https://github.com/backdrop/backdrop-issues/issues/5397.

Menu items have forever had the vague "Expanded" option, which affects the display of menu links when displayed in a hierarchical tree. Note that the "Expanded" option does not affect the drop-down menu display and can also be overridden using the "Show all menu links" option.

Proposed solution

Instead of having "Expanded" options at the menu link level, which only applies in very specific situations, move the settings for expanding menu links to the block level, and only show this setting when using the "Hierarchical Tree" display option:

The obvious downside of this approach would be that if you place the same menu block in multiple places on the site, you would need to reconfigure the list of expanded links multiple times, unless you used a contrib module like mini_layouts to reuse the same block configuration across multiple layouts.