fix: SlickCellExternalCopyManager should work w/hidden cols fixes #1634

[ghiscoding]

fixes #1634

• prior to this PR, the hidden columns were being counted as column indexes when pasting and also when starting a cell selection

in the example below, the column C and D are hidden

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.8%. Comparing base (559d7de) to head (7b48596).
Report is 14 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #1651     +/-   ##
========================================
- Coverage    99.8%   99.8%   -0.0%     
========================================
  Files         199     199             
  Lines       21874   21882      +8     
  Branches     7176    7040    -136     
========================================
+ Hits        21811   21818      +7     
- Misses         57      64      +7     
+ Partials        6       0      -6     

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