Summary:
This draws a line underneath a Plasma checkbox label when it has
keyboard focus. This is needed as when an item is checked, the focus
shadow is practically invisible.
Visuals when clicking is completely unchanged
This brings it more in line with our QStyle.
Test Plan: Attached runtime tests
Reviewers: #plasma, #vdg, mart
Reviewed By: #plasma, mart
Subscribers: plasma-devel, #frameworks
Tags: #plasma, #frameworks
Differential Revision: https://phabricator.kde.org/D7819
