Added Next/PrevTabKeys.

Refactored ApplicationNavigation in prep for further work

UnitTests/Application/ApplicationTests.cs

@@ -201,6 +201,9 @@ public class ApplicationTests
             // Keyboard
             Assert.Empty (Application.GetViewKeyBindings ());
 
+            // Navigation
+            Assert.Null (Application.Navigation);
+
             // Events - Can't check
             //Assert.Null (Application.NotifyNewRunState);
             //Assert.Null (Application.NotifyNewRunState);
@@ -241,6 +244,8 @@ public class ApplicationTests
 
         //Application.WantContinuousButtonPressedView = new View ();
 
+        Application.Navigation = new ();
+
         Application.ResetState ();
         CheckReset ();