Add Kernel.get_parent_header

[minrk]

and deprecate instead of break Kernel._parent_header.

While testing the 6.0 alpha, I noticed an issue with the OutputWidget, which references the parent header here. #585 changes _parent_header to be a nested dict of headers, now that two channels may have different parents, causing the output widget to fail with a KeyError on 'msg_id'.

This is ~understandable because widgets are using a private API, but at the same time there isn't a public API for getting the current parent header, which the output widget needs.

I went with:

• keep Kernel._parent_header to mean shell, but with a deprecation
• rename new still-private nested headers trait to _parent_headers
• add Kernel.get_parent_header(channel="shell") API with clearer stability expectations

This way the Output widget doesn't break, and has a clearer path to using a non-private non-deprecated API, which wasn't available before.

related to #635

[blink1073]

Thanks! Heads up, docstring linter picked up one thing

[minrk]

Ah, just noticed something weird - _parent_header wasn't actually the parent header, it is the parent message. We should probably fix that while we're at it before we publish a new public API with a name that doesn't make sense.