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

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