feat(sorting): header menu clear sort, reset sorting when nothing left

[ghiscoding]

• when there is no more column to sort, we could resort by default sort id which would display dataset the way it was when it was first loaded
• ref SO question: https://stackoverflow.com/questions/62489108/angular-slickgrid-column-wise-remove-sort-option-not-working-properly
• also found that Clear Column Sort on a Local Grid was not trigger a "sortChanged" event while it should so that the Grid State is also notified

[codecov]

Codecov Report

Merging #509 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##            master      #509    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files          148       148            
  Lines         9578      9586     +8     
  Branches      3202      3342   +140     
==========================================
+ Hits          9578      9586     +8     

Impacted Files	Coverage Δ
...lickgrid/components/angular-slickgrid.component.ts	100.00% <100.00%> (ø)
...ngular-slickgrid/extensions/headerMenuExtension.ts	100.00% <100.00%> (ø)
...modules/angular-slickgrid/services/sort.service.ts	100.00% <100.00%> (ø)

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