Customize the file browser toolbar via the settings

[jtpio]

Problem

Just like the notebook toolbar or file editor toolbar, it would be great if the file browser toolbar could also be configured via the settings.

Proposed Solution

Maybe we can follow a similar approach as the other factories, and place the toolbar items in the settings definition.

Additional context

This could also prove useful for customizing the notebook v7 toolbar (see https://github.com/jupyterlab/retrolab/issues/260#issuecomment-971202618 for more context):

[jtpio]

Adding to 4.0 for consideration.

[jtpio]

This might depend on #11754 being fixed first for use in Notebook v7.

[jtpio]

Looks like the current toolbar customization applies to the document factories.

Not sure it will be possible to adapt the current pieces such as createToolbarFactory so they work on generic widgets with a toolbar.

cc @fcollonval who might have some input on this.

[jtpio]

Ideally in the case Notebook v7 it would be possible to register a new factory like it is already done here for the interface switcher in the notebook toolbar:

https://github.com/jupyter/notebook/blob/30310847d6804294cbeb274eeb91d126c86669f0/packages/lab-extension/src/index.ts#L140-L152

And define overrides in a custom downstream settings file, for example to be able to make the toolbar look a bit more like classic if needed (disabling some items, inserting spacers to pull content to the right):

[jtpio]

One thing that could be tricky is the fact there can be multiple file browsers. It's not clear whether it will be possible to have something generic enough without making it too complicated.