Add CssBoxCollection class, optimize Search

1. CssBoxCollection Class: Collect and Manage CssBox Children
benefits:  ontrol Add,Insert,Remove, set parent to CssBox
2. Optimize Collection: Reduce number of times that need to seach
CssBox, in IndexOf or Contains method of List
3. Reorder  MemberItem in CssDisplay Enum : group CssDisplay props that
have Container Property together.
4. CssBox_Corrector.cs : clearify code in
GetPreviousContainingBlockSibling() and  GetPreviousSibling()