Fix ASSERT: "lister->d->m_cachedItemsJob == 0" when using KDirModel::expandTo(/a/b/c) and

/a and /a/b are in cache already; when the CachedItemsJob for /a emits its items,
KDirModel sees b and asks for listing /a/b, which creates another CachedItemsJob for /a/b.
So it's perfectly possible to have more than one cached items job in a dirlister, just
not for the same directory -> turned the job member into a list of jobs. With unittest.
FIXED-IN: 4.6.1
BUG: 175035
(cherry picked from commit 4530a699)