removing Java-ism on *MenuItemImpl.
First UI Control XMLization.
more control UI XMLization: BSplitContainer.
Some refactoring in BSplitContainerImpl and UiControl API.
remove some unused code from some controls.
Eliminated System.Drawing and System.Windows.Forms from BSplitContainerImpl.
Moved most of BSplitContainer UI implementation code to "model".

It is not really like a "model", but rather be like view model.
As the REFACTORING.md explains, the first purpose of this UI "model" is
to make things portable, so it goes along with that.