kcms/users: Port avatar buttons to Kirigami.ShadowedImage
This does proper treatment to source image item, and hides it for us.
Also using this opportunity to clean up property access code a little, because poking around random image types to get your source url back is no good for code modularity.
Requires Kirigami patch to add missing mipmap
property: frameworks/kirigami!1221 (merged)