Discovered serious issues with how HasFocus, OnEnter/OnLeave, etc... work in some edge cases.

This will require re-visiting the design at a deep level and fixing some long-standing but ignored issues such as how OnEnter/OnLeave don't follow proper cancelation design. Also, there's a need for keeping track of the old focus state of a tree of subviews when that tree loses focus; FocusDireciton is a hack that causes tons of confusion.

UnitTests/Views/TreeTableSourceTests.cs

@@ -289,7 +289,7 @@ public class TreeTableSourceTests : IDisposable
 
         var top = new Toplevel ();
         top.Add (tableView);
-        top.RestoreFocus ();
+        top.RestoreFocus (null);
         Assert.Equal (tableView, top.MostFocused);
 
         return tableView;