Dropdown menu: support checks, options, separators, and labels

[nielsbosma]

I want to be able to create a rich dropdown menu like GitHub's diff "Layout" settings dropdown using Ivy features. It combines several elements:

• Section labels (e.g. "Layout")
• Checkable items with a checkmark indicating the selected/active state (e.g. "Unified" / "Split", "Hide whitespace")
• Separators between groups of items

Question: Do we already have the API to build all of this (checks, options, separators, labels)? If not, what's missing?