KUriFilter: port away from KServiceTypeTrader
Summary:
Instead, locate the desktop files and read them, into a memory cache.
This allows to run the KIO unittests without installing KIO.
Test Plan:
kurifiltertest and kurifiltersearchprovideractionstest
now pass with empty XDG_DATA_DIRS.
Reviewers: apol, davidedmundson
Reviewed By: apol
Subscribers: Frameworks
Tags: Frameworks
Differential Revision: https://phabricator.kde.org/D9213