style(Dropdown): update typings and propTypes usage

[layershifter]

This PR is part of work for removing propTypes in production bundle (#524, #731).
Also, cleanups and updates typings for #1072.

[codecov-io]

Codecov Report

Merging #1334 into master will decrease coverage by -0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1334      +/-   ##
==========================================
- Coverage   99.75%   99.75%   -0.01%     
==========================================
  Files         140      140              
  Lines        2402     2401       -1     
==========================================
- Hits         2396     2395       -1     
  Misses          6        6

Impacted Files	Coverage Δ
src/modules/Dropdown/Dropdown.js	100% <ø> (ø)	✅
src/modules/Dropdown/DropdownItem.js	100% <ø> (ø)	✅
src/modules/Dropdown/DropdownHeader.js	100% <ø> (ø)	✅
src/modules/Dropdown/DropdownMenu.js	100% <ø> (ø)	✅
src/modules/Dropdown/DropdownDivider.js	100% <ø> (ø)	✅

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[levithomason]

Released in semantic-ui-react@0.66.0.

[wcatron]

The typescripts in this PR are too strict.

Property 'onClick' DropdownItemProps
Select uses the DropDown props which require onClose, onFocus, onMouseDown, etc.

There are others. Should I open an issue addressing all of these or do you they need to be individual?

[levithomason]

Please open a new issue or PR.