Publish a message when all images are published for a compose

[AdamWill]

Currently it's not possible for something to know "OK, fedimg has done publishing images for Compose X". We get a message every time a single image is published, but there's no way to know which was the last image for a given compose. If you just want to wait till all images are published then do (something), there's no good way to do that. It would be useful to have a message published that communicates this.