1927: Another technique for PartialBoxStrip

*** Another technique for PartialBoxStrip
1.  CssLayoutEngine: remove BubbleStripsUpdate() and use
     leaner and faster!  CssLineBox.CloseLine()  method
     (this reduces a number of search loop)

2.  CssLineBox:
     2.1 CloseLine() method create List<PartialBoxStrip>
     2.2 change type and name of _boxStrips  toList<PartialBoxStrip>
_bottomUpBoxStrips.
         not need Dictionary type, just use a List.
     2.3 change some loop code on that list