Added List base clss for MenuList

-Moved all functionality regarding caption, main rect, initialization
and scroll bar to List
-Removed all above functionality from MenuList and replaced it by
innheriting from List
-ScrollUp() and ScrollDown() is private pure virtual. This is because
	-Scrolling needs to be done on the induvidial items in the List
	( i.e. available games or configuration items )
	-The method CheckScrollBarIntersection in List uses these
	method, and this any derived class HAS to implement them.
	-Private because scrolling is ( for now ) only triggered by the
	scroll bar itself.
-Removed headers that are present in List from MenuList