[UX] Add a "Expand all" checkbox on menu block config form

[opi]

Describe your issue or idea

Right now, when using the "Hierarchical tree" style for menu block, menu tree is only expanded for active trail.

By adding a checkbox, it will allow site administrator to decide if they want to display every menu children, for example to create custom dropdown menu without using core's smartmenus library.

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PR by @opi: backdrop/backdrop#1932

[opi]

[jenlampton]

The code looks great on this @opi! I made a few suggestions on the PR for the language on the checkbox, but otherwise I think this will be a great improvement for user experience.

[jenlampton]

Thanks for the updated PR @opi, I'm marking this issue RTBC. I'm also going to add the milestone 1.7.2, but it's possible this could be classified as a new feature and bumped to 1.8.0 instead. Either way, it should go in soon!

[klonos]

Thank you @opi 👍 Great job!!