Returns the cell width of the "prototypical cell" -- a cell used
for the calculation of cell widths, because it has the same value
as all other list items, instead of forcing the calculation to
inspect every item in the list.
If this value is non-null it's used to compute fixedCellWidth
and fixedCellHeight by configuring the cellRenderer at index equals
zero for the specified value and then computing the renderer components
preferred size.
Sets the data model's isAdjusting property true, so that
a single event will be generated when all of the selection
events have finished (for example, when the mouse is being
dragged over the list in selection mode).