Provide a way to easily query which feedback options are available

Summary:
This will be useful to create better cross-process tools that expose the
required feedback information.

Test Plan:
Implemented it in Plasma Shell and Discover, adopted the results in
the Feedback KCM for showing who uses what.

Reviewers: Plasma, vkrause

Reviewed By: vkrause

Subscribers: ngraham

Differential Revision: https://phabricator.kde.org/D26569