Introduce an "event bus"-like system. It's more like a fan-out
broadcaster for certain events. Instead of directly sending events to
SocketIO, they are now sent to the broker, which in turn sends it to any
registered "forwarder". Currently there is ony one forwarder, for
SocketIO.
This opens the door for a proper MQTT client that sends the same events
to an MQTT server.
Rename the add-on from `flamenco3-addon.zip` to `flamenco-addon.zip`.
It still contains the same files as before, and in Blender the name of
the add-on has not changed.
Change the package base name of the Go code, from
`git.blender.org/flamenco` to `projects.blender.org/studio/flamenco`.
The old location, `git.blender.org`, has no longer been use since the
[migration to Gitea][1]. The new package names now reflect the actual
location where Flamenco is hosted.
[1]: https://code.blender.org/2023/02/new-blender-development-infrastructure/
Extract some code from `cmd/flamenco-manager/main.go` into `webservice.go`
in the same directory, just to make `main.go` a little smaller.
No functional changes.
