fix(composite): onSave always include last dataContext on few inserts

[ghiscoding]

• fixes issues reported in Angular-Slickgrid: Incorrect dataContext is passed in onSave callback using composite editor Angular-Slickgrid#1333
• prior to the fix, only the first item inserted had the correct dataContext argument provided to the onSave, any other inserts were resolving to another unrelated data context
• to fix this issue, we'll get the updated item right after it's been called by the onAddNewRow SlickEvent, which is the event being called after all editors are valid. Note however that an inserted item can also have an optional insertNewId which is taken care of with this fix as well

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (701da75) 86.71% compared to head (76d2ab8) 86.71%.

Files	Patch %	Lines
...-component/src/slick-composite-editor.component.ts	90.91%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1271      +/-   ##
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- Coverage   86.71%   86.71%   -0.00%     
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  Files         197      197              
  Lines       21374    21377       +3     
  Branches     7103     7107       +4     
==========================================
+ Hits        18532    18534       +2     
- Misses       2842     2843       +1     

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