[Lock, login & logout screen] Add visual feedback to action buttons when pressed

Summary: The action buttons were missing a visual indication of when they were being pressed so this patch adds it by drawing another an animated rectangle when the button is pressed.

Test Plan: {F6819154}

Reviewers: #vdg, #plasma, ngraham, davidedmundson

Reviewed By: #vdg, #plasma, ngraham, davidedmundson

Subscribers: broulik, abetts, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D21062