Fix avatar picture aliasing and stretching in kickoff

Summary:
QML Image element has some smoothing when scaling an image by default but it's not enough and the picture still looks a bit pixelated. This patch fixes that.
Also while I was at it, I added keeping the aspect ratio, because no one likes their avatars stretched.

BUG: 369327

Test Plan:
Before: {F5819424} {F5819425}
After: {F5819429} {F5819430}

Reviewers: #plasma, mart

Reviewed By: #plasma, mart

Subscribers: davidedmundson, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D12469