Introduce a QCommandLineParser for kglobalaccel5

Summary: It's useful to be able to check the version and so.

Test Plan: Running kglobalaccel5 --help doesn't execute the service.

Reviewers: Frameworks, davidedmundson

Reviewed By: davidedmundson

Subscribers: graesslin

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D4175