avatar: Consider deprecating the size property
The size property of HdyAvatar
always felt weird to me, that's not how sizes are typically handled except for some corner cases like GtkImage, and even for it it felt like a weird decision.
Following !661 (merged) and !662 (closed), I'd suggest deprecating the size property and using CSS instead. we probably instead should let the developer choose the size request mode potentially via an orientation.