[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.

ObservatoryFramework/Interfaces.cs

@@ -187,6 +187,13 @@ namespace Observatory.Framework.Interfaces
         /// <param name="items">Grid items to be added. Object types should match original template item used to create the grid.</param>
         public void AddGridItems(IObservatoryWorker worker, IEnumerable<object> items);
 
+        /// <summary>
+        /// Replace the contents of the grid with the provided items.
+        /// </summary>
+        /// <param name="worker">Reference to the calling plugin's worker interface.</param>
+        /// <param name="items">Grid items to be added. Object types should match original template item used to create the grid.</param>
+        public void SetGridItems(IObservatoryWorker worker, IEnumerable<object> items);
+
         /// <summary>
         /// Clears basic UI grid, removing all items.
         /// </summary>