[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