OutputManager: When listeners are registered, they need to be notified of existing globals

[AlanGriffiths]

In particular, WindowWlSurfaceRole::output_global_created() uses the notification to register as an observer on the global. Without this, it cannot track configuration changes

[mattkae]

This change makes sense to me.

However, this does seem like a minor paradigm shift with our listeners. For example, XdgOutputV1 is an OutputConfigListener who listens on output_config_changed. When we add the listener (in XdgOutputV1's constructor), we don't automatically trigger the event for the listener, but rather allow the listener to manually trigger its own event.

It might be surprising that some listeners trigger events on listen, while others do not. But that's my 2c :)

[AlanGriffiths]

However, this does seem like a minor paradigm shift with our listeners.

I'm sure there are both other registrations that do this and other registrations that don't. I guess it depends on what is being notified - in this case, I don't see a downside.

[hbatagelo]

Looks good to me.