Draft: Implement placeholder mechanism to show avatars

Introduce a mechanism to display profile avatars and group logos. This MR is hovering somewhere between "a good start" and "a bad idea".

• Create SettingsBridge.stringValue("avatar_dir")
• Groups: use group ID as filename
• Recipients: use UUID as filename
• Update relevant QML files accordingly

This is a very much work in progress, because:

• Recipient profile avatar field isn't used
• Groups don't really have an avatar field?
• Recipient UUID is exposed to filesystem
• Group ID is exposed to filesystem

It should be easy to update this MR to use recipient/group avatars when the backend work is done. The UI side should be complete, though. I have manually copied a few images to avatars folder, and they show up properly.