Improve Plasma user feedback introspection

Summary:
Before loading the view, query all plasma processes and show why they're using
KUserFeedback.

Depends on D26569

Test Plan: Manual testing

Reviewers: #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: ngraham, davidedmundson, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D26570