[Core + Framework] Performance improvements for re-drawing the grid (#141)

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.

ObservatoryCore/UI/PluginHelper.cs

@@ -61,6 +61,7 @@ namespace Observatory.UI
             {
                 Anchor = AnchorStyles.Left | AnchorStyles.Right | AnchorStyles.Bottom | AnchorStyles.Top
             };
+            plugin.PluginUI.UI = panel;
 
             IObservatoryComparer columnSorter;
             if (plugin.ColumnSorter != null)