Reshape Kickoff avatar to a circle with Opacity Mask

Summary:
Crop the square user avatar into a circle with an outline, like in SDDM and
the lock screen.

BUG:  386656
FIXED-IN: 5.18.0

Test Plan:
- Use Kickoff
- Change/set avatar in User Manager
- See pretty circle-cropped avatar

{F7777572, size=full}

Reviewers: sharvey, #vdg, #plasma, mart

Reviewed By: #vdg, #plasma, mart

Subscribers: filipf, sefaeyeoglu, hein, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D13415