Project tree: compare all items case-insensitively

The commit history is silent about the reason why folders and targets
are sorted case-sensitively, while files - case-insensitively. Sorting
all items case-insensitively is more consistent and potentially faster
as std::stable_sort used by QSortFilterProxyModel could start with a
closer-to-sorted range then.
1 job for !223 with project-tree-compare-all-ci in 33 minutes and 54 seconds (queued for 44 minutes and 26 seconds)
detached
Status Job ID Name Coverage
  Build
passed #91732
linux kf5-qt5 qt5.15

00:33:54

46.0%