[avatar] Reverse the visual stacking order of avatars in a group

[jouni]

Make it possible to reverse the visual stacking order of grouped avatars.

This can be a choice some design systems want to make, that avatars are stacked in certain way, perhaps depending on the situation. The stacking order gives more visual prominence to the top-most avatar, and can therefore support certain use cases better than the alternative stacking.

This would provide a workaround to vaadin/vaadin-avatar#87

HTML+CSS prototype

I’m not sure if Avatar Group already does this for RTL languages. But it should work the opposite way there, obviously.