Make Dropdown grouping more explicit

[Anders2303]

PR #794 introduced a feature in Dropdown that allows you to group dropdown-options together under a label in the scroll-menu. How-ever, the current approach does not guarantee that options of the same group are shown together if the options array is not properly sorted before-hand, making implementation cumbersome and less intuitive.

As suggested by @rubenthoms in #794 (comment), we should consider having two types of options - e.g. DropdownOption and DropdownGroup, where the group wraps a list of options.

Naturally, this issue is only relevant if #794 is merged, but this issue should be prioritized before too many implementations make use of the current solution