refactor: convert Dropdowns to FunctionComponent and wrap with forwardRef

[alexpaxton]

Closes #291

Changes

• Organize /Dropdowns folder:
  • Create /Documentation for docs related files
  • Create /index.tsx for family rollup class
  • Move Dropdowns/Family contents into /Dropdown
• Convert all family and composed components to FunctionComponent and wrap with forwardRef
• Create and export prop and ref types for all dropdown components
• Add ref logger widget to stories for all dropdown components
• [Breaking] remove widthPixels prop from Dropdown, SelectDropdown, and MultiSelectDropdown in favor of a default value for the style prop

Checklist

Check all that apply

• Updated documentation to reflect changes
• Added entry to top of Changelog with link to PR (not issue)
• Tests pass
• Peer reviewed and approved
• Signed CLA (if not already signed)

[alexpaxton]

I did not add a ref for DropdownButton since it consumes ButtonBase which does not yet have a ref. Waiting on #285 before merging

[mavarius]

Good to go