fix(TreeData): addItem should keep current sorted column

[ghiscoding]

• prior to this PR, calling addItem(), or addItems(), was always resorting using the Tree Data initial sort because the method being called is the same for the first build of the Tree Data and/or adding an item. So when we're adding an item, we should really reuse any existing sort that may exist when resynching the flat/hierarchical and resorting them (we always need to resort whenever it changes because by SlickGrid doesn't support Tree Data, we only support it by keeping copies of both flat & hierarchical and then resort them whenever something changes)

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Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.8%. Comparing base (7d4a769) to head (2fc3e50).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1558 +/- ## ======================================== + Coverage 99.8% 99.8% +0.1% ======================================== Files 198 198 Lines 21722 21723 +1 Branches 7270 7271 +1 ======================================== + Hits 21661 21662 +1 Misses 55 55 Partials 6 6 ```

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