Display exclusive actions correctly in systray menus

Summary:
Add exclusive actions to their own action groups so that they
could be diplayed as radio buttons by styles. It's only to
display them correctly, the switching logic is done by Qt
outside this plugin.

Reviewers: Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: davidedmundson, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D9733