fix(Dropdown): sets focus to the search input after selection

[jongsue]

Addresses #3349.

[codecov-io]

Codecov Report

Merging #3430 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #3430   +/-   ##
=======================================
  Coverage   99.92%   99.92%           
=======================================
  Files         172      172           
  Lines        2819     2819           
=======================================
  Hits         2817     2817           
  Misses          2        2

Impacted Files	Coverage Δ
src/modules/Dropdown/Dropdown.js	100% <100%> (ø)	⬆️

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[layershifter]

I need to merge #3428 before.
Tried the similar fix today, but there were issues with broken key handlers.

[layershifter]

I added some small changes. Actually, this PR don't fix #3349 because the focus will be still lost on usual <Dropdown selection />. Going to merge this PR because it fixes the issue for <Dropdown search selection />.

[levithomason]

Released in semantic-ui-react@0.86.0.