Include focus mixin in focusable widgets

[smhigley]

Type: feature

The following has been addressed in the PR:

• There is a related issue
• All code matches the style guide
• Unit or Functional tests are included in the PR

Description:

Related to #342, makes it possible to focus existing widgets using the Focus mixin

[codecov]

Codecov Report

Merging #608 into master will increase coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #608      +/-   ##
==========================================
+ Coverage   98.96%   98.99%   +0.03%     
==========================================
  Files          42       42              
  Lines        3081     3080       -1     
  Branches      832      830       -2     
==========================================
  Hits         3049     3049              
+ Misses         32       31       -1

Impacted Files	Coverage Δ
src/radio/index.ts	100% <100%> (ø)	⬆️
src/tab-controller/index.ts	100% <100%> (ø)	⬆️
src/slider/index.ts	100% <100%> (ø)	⬆️
src/title-pane/index.ts	100% <100%> (ø)	⬆️
src/select/index.ts	100% <100%> (ø)	⬆️
src/time-picker/index.ts	100% <100%> (ø)	⬆️
src/text-area/index.ts	100% <100%> (ø)	⬆️
src/checkbox/index.ts	100% <100%> (ø)	⬆️
src/listbox/index.ts	100% <100%> (+0.81%)	⬆️
src/tab-controller/TabButton.ts	100% <100%> (ø)	⬆️

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