Global log console for custom messages

[jtpio]

The reason for having a separate log console for custom messages comes from the fact that the default log console in JupyterLab tracks notebook widgets:

https://github.com/jupyterlab/jupyterlab/blob/6eec51b81635b6c379b623e3c53cc3589020a111/packages/logconsole-extension/src/index.tsx#L274-L278

This means that the log console is then disabled when the focus is on different type of widget:

In the end this currently called jupyterlab-js-logs extension could actually be a more "global log console" that other plugins could consume to post custom messages.