Allow mulitple launchers to different paths for a single server

[bollwyvl]

Proposed change

Allow a single server instance to open different URLs with different labels, icons, etc.

Even if a single path is given, it can be nice to pre-seed it with a specific path, including ? params and # fragments (#222). fixed by #279

Alternative options

Using multiple servers is unsatisfying if the route in question offers a number of different endpoints, each of which might be relevant, or in the case of Lab, might be interesting to have open simultaneously.

Some applications aren't particularly well-behaved (especially when run under a proxy) and need a little help to know where they are.

Who would use this feature?

• the dask dashboard has a number of different views that can be launched
• jupyterlite offers a number of applications (lab, classic) that can "talk" to each other and it would be nice to have a single server to run them

(Optional): Suggest a solution

In config, allow launcher_entry to be an array, requiring a name for each entry (to build predictable icon URLs). Probably also allow new_browser_tab to be set per-launcher, defaulting to the root one.

In the frontsend, a multi-launcher as a menu could have submenus:

JupyterLite >
  Lab
  Classic
  Book
Dask >
  Task Stream
  Profiler
  ...

Meanwhile, in the Lab launcher would draw multiple cards, command palette entries, etc.

[bollwyvl]

#267 is up with a proposed implementation.

[bollwyvl]

The singleton case of this is fixed by #279. Will update #267 at some point in a way that's compatible with this.