[libtaskmanager] Go back to using KServiceTypeTrader for looking up desktop entries
4f53ed33 changed two things:
It removed the NoDisplay check
It ported from KServiceTypeTrader to KService::fromDesktopEntry
However the latter caused a behavior change that broke mapping some applications.
Previously an appId of "Sublime_merge" would be mapped to sublime_merge.desktop, but the new code doesn't take mixed casing into account
Revert that part