KDirModel: fix hasChildren() regression for trees with files shown
The logic I introduced in commit ebe7714300af53ba6f4b5cacddb53a2fd52c7d5a
assumed that only subdirs were shown in the tree (konqueror sidebar use
case). However KDirOperator can also show files, so this introduced bug
- Now implemented correctly for both use cases, with unittest.
BUG: 419434
FIXED-IN: 5.71