Way too big a committ.... but I got carried away.

- Started MouseEventEventArgs -> MouseEvent transition - for OnMouseEvent. Partially fixes #3029.
- Refactored Appliation.OnMouseEvent and View.OnMouseEvent to match design guidelines.
- Re-impleented highlight, enabling extensibility (see Border).
- Beefed up unit tests

Terminal.Gui/ConsoleDrivers/CursesDriver/CursesDriver.cs

@@ -812,7 +812,7 @@ internal class CursesDriver : ConsoleDriver
         var me = new MouseEvent { Flags = mouseFlag, X = pos.X, Y = pos.Y };
         Debug.WriteLine ($"CursesDriver: ({me.X},{me.Y}) - {me.Flags}");
 
-        OnMouseEvent (new MouseEventEventArgs (me));
+        OnMouseEvent (me);
     }
 
     #region Color Handling