Add support for rank to the top area

[jtpio]

References

Partially related to #6096.

#6096 is more general and also considers the main area.

This change is only for the top area.

Code changes

Add a PanelHandler wrapper to store the widget ranks for the top area.
It should be possible to use the same wrapper for the header and bottom areas too, and potentially merge it with the existing SideBarHandler (also sorting its widgets).

Ideally all areas should support ranks by default. However as pointed out by @Madhu94 in #6096 this might become tricky for the main area (is there a notion of rank / order for this area?)

Developer-facing changes

Third-party extensions (example) can now use the rank parameter when adding widgets to the top area:

// place a custom widget in front of the Jupyter logo
app.shell(widget, 'top', { rank: -1 });

User-facing changes

None

Backwards-incompatible changes

None

[jupyterlab-dev-mode]

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[blink1073]

LGTM, thanks!