[Lock, Login, and Logout screen] Adjust visual feedback of action buttons

Summary:
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.

BUG: 393048

Test Plan:
Logout:
{F6664346}

Lock:
{F6664347}

Login:
{F6664348}

Reviewers: rooty, ngraham, #vdg, #plasma

Reviewed By: rooty, ngraham, #vdg

Subscribers: davidedmundson, plasma-devel

Tags: #plasma

Maniphest Tasks: T10325

Differential Revision: https://phabricator.kde.org/D19479