2014: refactor

move some internal-used methods to CssBoxExtension
and make it an extension method
2016: Remove First and Last HostingLineBox

1. Remove FirstHostingLineBox and LastHostingLineBox Properties (and
fields)
from CssBox,
2. Use GetSplitInfo() method of CssBoxExtension to test if a CssBoxLine
is
first host line or last host line of that CssBox
2017: Remove a CalculateWidthMarginRecursiveUp()

Remove CssBox. CalculateWidthMarginRecursiveUp() -> this is a recursive
method,
then Use LayoutArgs. CalculateWidthMarginTotalUp() instead (use
accumulation of total margin)
2018: fix
Merge branch 'v1.7'