TreeMap: fix compare function

For descending sorting, TreeMapItemLessThan
implemented <=, not <.

qSort obviously has no problem with that, but
in preparation of replacing the obsolete qSort
with std::sort, we need to fix it, because
it otherwise it can crash (actually observed!).
std::sort is known to be sensitive to non-strictly
ordering compare functions, eventually resulting
in out-of-bounds accesses.