Split Table._setScrollbarWidth by adding public function Table.getScrollbarWidth

[RoosDaniel]

For uses where a custom (external) header-row has to be used, there's little support for matching its width to the Table's width. Most of it can be circumvented by summing Column widths, but there's no way to get the scrollbarWidth used since the method _setScrollbarWidth does both the calculation and setState parts.

With this change, implementations that rely on matching this repo's internal _setScrollbarWidth function with an external (almost identical) function could instead use getScrollbarWidth.

[codecov-io]

Codecov Report

Merging #1411 into master will decrease coverage by 0.04%.
The diff coverage is 87.5%.

@@            Coverage Diff             @@
##           master    #1411      +/-   ##
==========================================
- Coverage    90.4%   90.35%   -0.05%     
==========================================
  Files          59       59              
  Lines        1740     1742       +2     
==========================================
+ Hits         1573     1574       +1     
- Misses        167      168       +1

Impacted Files	Coverage Δ
source/Table/Table.js	88.39% <87.5%> (-0.7%)	⬇️

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[wuweiweiwu]

nice!

[wuweiweiwu]

minor release