Fix layout-issues when sorting by roles that are changed

When sorting is enabled for a role that gets changed e.g. because
a value like the MIME-type is determined asynchronously, then the
layout might get messed up.

slotItemsMoved() has been adjusted to invalidate the sizehint-cache
and to update the group-headers.

BUG: 285542
FIXED-IN: 4.8.0