* use the same method everywhere to display an avatar
* move code to generate letter avatars to a separate crate
* use the generated avatar as a fallback till the real avatar is
avaibile
* remove gtk dependecies from fractal-matrix-api

Note: The avatar in room details dialog is borken but the dialog will be
replaced soon. Also updating the username in the room history doesn't
work, the room history needs a refactor to resolve some issues.