Missing semantics in the structure of the option groups in the slash-dropdown

I partially fixed the problem with https://github.com/xwiki/xwiki-platform/commit/85de21b322b1c50572d41b93a057ec880af4d779 but on the semantic level the slash options should be inside the groups. ATM all groups are empty (the groups are on the same level as the options, acting more as separators). Fixing this breaks the keyboard navigation that we "inherit" from the CKEditor's Autocomplete feature (when you press Down key it looks for the next sibling option, it doesn't go recursively inside the groups). We'll have to overwrite this behavior.

Note that the HAN accessibility audit highlighted that the dropdown is not properly read out by screen readers when navigated. This poor structure might be the cause of this.