fix(pinning): recalculate frozen idx correctly when column shown changes

[ghiscoding]

• when using ColumnPicker, GridMenu or hiding a column via HeaderMenu, it has to recalculate the frozenColumn index because SlickGrid doesn't take care of that and the previous fix that I implement sometimes become out of sync. This PR simplifies the frozenColumn index position, it will simply update it when the index is different, as simple as that.

[codecov]

Codecov Report

Merging #682 (c6b7bd1) into master (7b95401) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##            master      #682    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files          157       157            
  Lines        10458     10451     -7     
  Branches      3515      3665   +150     
==========================================
- Hits         10458     10451     -7     

Impacted Files	Coverage Δ
...ular-slickgrid/extensions/columnPickerExtension.ts	100.00% <100.00%> (ø)
...s/angular-slickgrid/extensions/extensionUtility.ts	100.00% <100.00%> (ø)
.../angular-slickgrid/extensions/gridMenuExtension.ts	100.00% <100.00%> (ø)
...ngular-slickgrid/extensions/headerMenuExtension.ts	100.00% <100.00%> (ø)

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