Possible bug in settings changed listeners

[mbektas]

While working on (#5990), I noticed a possible reference issue with settings listeners. I saw it in couple of places but here is one example:
https://github.com/jupyterlab/jupyterlab/blob/master/packages/markdownviewer-extension/src/index.ts#L96
In this line updateSettings(settings); call is made using the settings argument from line 94.

I think it should be used as in https://github.com/jupyterlab/jupyterlab/blob/master/packages/statusbar-extension/src/index.ts#L106-L108 so that updated settings are applied.

Please look into this to decide if it is a real issue.

[jasongrout]

After some discussion, we decided not to dictate a style for this at this point. It is functioning properly (it's using the closure). If you feel strongly about it, we wouldn't object to a PR either.