Rename IApplication.Current to TopRunnable

Co-authored-by: tig <585482+tig@users.noreply.github.com>

Tests/TerminalGuiFluentTesting/FakeDriver/FakeApplicationLifecycle.cs

@@ -13,7 +13,7 @@ internal class FakeApplicationLifecycle (CancellationTokenSource hardStop) : IDi
     {
         hardStop.Cancel ();
 
-        Application.Current?.Dispose ();
+        Application.TopRunnable?.Dispose ();
         Application.Shutdown ();
     }
 }