Workaround for failed comparison of items in KFileItemModelRolesUpdater

The real fix is in the KDE/4.10 branch of kdelibs
(commit b8f64ca3f4b6311519c21046031d66d9d0a570c6). We work around the
problem that the KFileItem that a preview has been requested for is
different from the KFileItems in the sets m_pendingVisibleItems and
m_pendingInvisibleItems by refreshing those sets when we detect that
the bug has been triggered.

BUG: 304986
FIXED-IN: 4.9.3