[NUI] Add TouchArea property.
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.
This reverts commit
034889141391985f0ad1727ae6e32719d9c5a360.