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/View/NavigationTests.cs

@@ -510,7 +510,7 @@ public class NavigationTests (ITestOutputHelper _output) : TestsAllViews
                                      Assert.False (win.HasFocus);
 
                                      win.Enabled = true;
-                                     win.FocusFirst (null);
+                                     win.FocusDeepest (null, NavigationDirection.Forward);
                                      Assert.True (button.HasFocus);
                                      Assert.True (win.HasFocus);