fix(core): showing/hiding column shouldn't affect its freezing position

[ghiscoding]

• when calling "hide column" we need to readjust the freezingColumn by (-1) when on left container
• when showing/hiding a column from ColumnPicker/GridMenu, we need to check if we need to also readjust when column is on the left container

TODOs

• add all Jest unit tests
• add Cypress E2E test
• requires core SlickGrid PR #561 to be merged & released

[codecov]

Codecov Report

Merging #642 (7907cb8) into master (13809b4) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##            master      #642    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files          154       154            
  Lines        10301     10333    +32     
  Branches      3614      3478   -136     
==========================================
+ Hits         10301     10333    +32     

Impacted Files	Coverage Δ
...lickgrid/components/angular-slickgrid.component.ts	100.00% <100.00%> (ø)
...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)
...dules/angular-slickgrid/services/shared.service.ts	100.00% <100.00%> (ø)

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