fix(core): Flatpickr is not destroyed properly & leaks detached elements

[ghiscoding]

• 2nd pass to fix #625

[codecov]

Codecov Report

Merging #627 (6f07141) into master (d1e284f) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##            master      #627    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files          149       149            
  Lines        10048     10062    +14     
  Branches      3391      3544   +153     
==========================================
+ Hits         10048     10062    +14     

Impacted Files	Coverage Δ
...lickgrid/components/angular-slickgrid.component.ts	100.00% <ø> (ø)
...pp/modules/angular-slickgrid/editors/dateEditor.ts	100.00% <100.00%> (ø)
...es/angular-slickgrid/filters/compoundDateFilter.ts	100.00% <100.00%> (ø)
...dules/angular-slickgrid/filters/dateRangeFilter.ts	100.00% <100.00%> (ø)
...es/angular-slickgrid/services/extension.service.ts	100.00% <100.00%> (ø)
...pp/modules/angular-slickgrid/services/utilities.ts	100.00% <100.00%> (ø)

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