fix(Checkbox): preventDefault() in mouseDown should prevent focus

[layershifter]

Fixes #3402.

Checkbox component should behave as native input[type=checkbox]. preventDefault() in onMouseDown should prevent Checkbox from focusing:

With changes in this PR, the code below now works properly:

<Checkbox onMouseDown={(e) => e.preventDefault()} />

[codecov-io]

Codecov Report

Merging #3732 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3732      +/-   ##
==========================================
+ Coverage   99.83%   99.83%   +<.01%     
==========================================
  Files         175      175              
  Lines        3121     3122       +1     
==========================================
+ Hits         3116     3117       +1     
  Misses          5        5

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

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