[Lock, Login, and Logout screen] Adjust visual feedback of action buttons
Unlike in the logout screen, our action buttons in the SDDM theme currently offer no visual feedback when interacted with. This patch adds a minimal yet effective amount of opacity shifting, as well as a transparent circle behind the button. The same circle is added to logout screen, where it remains always visible to better indicate the chosen option.
Reviewed By: rooty, ngraham, VDG
Subscribers: davidedmundson, plasma-devel
Maniphest Tasks: T10325
Differential Revision: https://phabricator.kde.org/D19479