This proposes a *new method* on IObservatoryCore: `SetGridItems(worker, items)` which does 2 things:
* Clears the grid
* Adds the given items.
Effectively replaces the entire content of the grid -- which is something a handful of plugins now do.
Why add this when you could just call Core's Clear + AddGridItems methods?? So it can all be done within the same rendering suppression "scope" to reduce flickering and increase rendering speed. Speaking of rendering suppression, I have implemented such rendering suppression "scope" which uses Listview's built-in Begin/EndUpdate() in combination with temporary removal of the sort comparer (as is done for read-alls). This was also applied to the existing AddGridItems(worker, items) method as well, addressing a TODO.
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