1.[NUI 276] c# nui visual high level class refactorying.

1.[NUI 276] c# nui visual high level class refactorying.

2.Put some common enums and structs into a NUIConstants file.

Note:
HorizontalAlignment.HorizontalAlignBegin --> HorizontalAlignment.Begin,
HorizontalAlignment.HorizontalAlignCenter --> HorizontalAlignment.Center,
HorizontalAlignment.HorizontalAlignEnd --> HorizontalAlignment.End,
VerticalAlignment.VerticalAlignTop --> VerticalAlignment.Top,
VerticalAlignment.VerticalAlignCenter --> VerticalAlignment.Center,
VerticalAlignment.VerticalAlignBottom --> VerticalAlignment.Bottom.

Change-Id: I770900db2404cb46b29269853e1db3f37e7e5401