Add a way to determine event source from onChange event

[aleqsio]

We've encountered a usecase where we needed to handle onChange event differently depending if the click originated from a label or a slider element. This fork adds a source to the onChange argument dictionary, that takes values keyboard|label|slider and allows to handle such a usecase.

[codecov[bot]]

Codecov Report

Merging #578 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #578   +/-   ##
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  Coverage   95.18%   95.18%           
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  Files           2        2           
  Lines          83       83           
  Branches       25       25           
=======================================
  Hits           79       79           
  Misses          4        4

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[haxxxton]

@aleqsio any chance you could amend your PR to only include the changes you intended to commit rather than ones that have been changed from something like prettier (quotes, line breaks etc.)?

[yoyo837]

Rebase please.