servicemenuinstaller: Use QMimeDatabase instead of running xdg-mime

Test Plan: ./test_run.rb still passes all tests.

Reviewers: elvisangelaccio, sitter, ngraham

Reviewed By: elvisangelaccio, ngraham

Subscribers: kfm-devel

Tags: Dolphin

Differential Revision: https://phabricator.kde.org/D22465