Added methods to convert `gkeepapi.node.Note` to `gkeepapi.node.List` and `gkeepapi.node.List` to `gkeepapi.node.Note`

[kyrlon]

Fixes kiwiz/gkeepapi#122. Added methods roughly outlined in the issue

[kiwiz]

Thanks for the PR. The (official) clients preserve the ID when converting between List/Note. IIRC, that was what made implementing this feature complicated.

The code needs to:

• Convert the body ✅
• Preserve the metadata
• Replace the existing List/Note that the Keep instance knows about

The last item is tricky as individual Nodes don't contain a reference to the Keep instance. This is why ListItems aren't visible to the Keep until they've been synced.

Our options:

• Use the implementation in the PR and add a note to "Known Issues".
• Refactor the code to inject a reference to the Keep instance within every Node.

[kyrlon]

Interesting. The latter option sounds like a challenge, but with your guidance I am up for it. Would this be some variant of dependency injection?

[kiwiz]

Hmm, perhaps we can get around this by making it a method on the Keep object like so:

list_ = keep.to_list(note)
note = keep.to_note(list)

That looks kind of odd though. I think the crux of the issue is: Does a user expect a converted Note/List to retain all its metadata?

[kyrlon]

Hmm that does look peculiar. Is it possible to make the reference to the keep instance optional?