Add files to pipelines through file browser context menus

[marthacryan]

Fixes #817. Shows a new option in the context menu of notebook files.

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[ajbozarth]

Other than one nit this LGTM, I ran it locally and this is great. I do have a handful of gripes I'll share below, but this deals with them as best as can be already.

• I don't like having to pass the signal up and down between the extension and widget, but even though we could move the code into the component class to minimize this it would mean every instance of a pipeline editor would try adding the command, which would probably not work as expected.

• I feel like we should distinguish the item in the context menu with a better icon, but any icon other than + wouldn't be a good fit for the action it performs, so + will have to work.

• I'd rather have the context menu only show if the editor is open, but that may be too difficult to implement and there already another context menu item that is also present even when inapplicable and behaves the same way as this (Shut Down Kernel).

[karlaspuldaro]

Agree with Alex's points noted, but I think this is a great enhancement as is for now :)