Add setting to expand menu items on hover

@ressa
I think Maintainers will decide whether will they provide this feature in the theme setting or not. But if you want a quick solution from overwriting then I can help you.

Thanks for the offer @ravi kant, but as you state, it's probably best if the maintainers agree to add this feature, or not.

Since I created this issue, https://www.drupal.org/project/disclosure_menu/ → is getting very close to being an alternative menu solution, where it's solved by having two menus - one for desktop, and one for mobile -- with different settings.