Make some VirtualFlow method protected

If someone wants to override the behavior of the VirtualFlow in a certain manner, he will need to override three methods:
-AddLeadingCells()
-AddTrailingCells()
-setCellIndex

Those should then become protected, and all the variable and methods inside should also become protected (accessible at least, no need to override those), that is to say:
-getCellLength
-getCellBreadth
-getAvailableCell
-positionCell
-resizeCellSize
-variable "maxPrefBreadth" (setter)
-getCellPosition
-variable "viewportLength" (getter)