[NUI] Add TouchArea property. (#2105)

[NUI] Add TouchArea property. (#2105)

Default touchable area is view's size.
TouchArea can reset the view's touchable area.

This is usefull when the view is small, but it should have a larger touch area.

for example

      View view = new View();
      view.Size = new Size(10, 10);
      view.TouchEvent += OnTouchEvent;
      view.TouchArea = new Size2D(200, 200);

The view is small, If you want to set the touch area to a larger area,
you can use the TouchArea property.