Currently, KStandardItemListWidgetInformant::itemSizeHint() calls the
model's data(int) method for every single item, but the full data is
actually only needed for the size calculation in Compact View. In
Details View, no data is needed at all to determine the size required
for the item, and in Icons View, only the name is needed.

This patch makes it possible for subclasses of
KStandardItemListWidgetInformant to provide an alternative way to
obtain the "text", and implements this in the subclass
KFileItemListWidgetInformant.

The final goal is to achieve that the QHash which contains all data
for a file item is only created if it is really needed, e.g., because
the view needs access to the data for displaying the item on the
screen.

REVIEW: 112253