Allow individual comments to be marked as draft

[hermannloose]

This is a proposal for #171166.

The names are rough suggestions, I am not sure if "visibility" is more descriptive than e.g. CommentDraftState.

CommentThread.hasDraftComments is included to show how we would conceptually want this to roll up into the thread. The surrounding features suggested in #171166 all operate on threads, but hasDraftComments does not necessarily need to become part of the API surface, it could be implicit and the relevant parts like line gutter decorations could just do the same iteration ad-hoc.

Staying with the example of line gutter decorations for illustration, our ideal UX would be:

• resolved thread: $(comment)
• unresolved thread: $(comment-unresolved)
• thread with a draft, regardless of status: $(comment-draft)

Note that in this particular case for the UI, it ends up looking like the "draft" state is mutually exclusive with resolved or unresolved CommentThreadState, but they are separate things, e.g. there could be two icons for lines that have a thread (or threads) with drafts, one icon to give the state, one icon to indicate presence of drafts.

[hermannloose]

cc @JonasMa @laurentlb @gkatja

[hermannloose]

Any initial thoughts on the proposal, regarding naming or structure?

[alexr00]

@hermannloose I haven't had a chance to give this the review it deserves. I will plan to review it during our next iteration (starts in 1.5 weeks).

[hermannloose]

I dropped hasDraftComments as per #171166 (comment).

visibility vs state did not sound as decided yet, so I kept it in for now. I think visibility is more descriptive than state here, so personally I would still lean towards visibility—also because it avoids potential API user confusion around CommentState just being CommentThreadState but at the Comment level; as mentioned above, they are separate and I would prefer the more obvious difference in name.

No strong opinions on Pending vs Draft but that also sounded like it was still up for debate, so I left it as is for now.

[alexr00]

With the visibility to state change I would accept a complete PR to add the published/draft feature.

[alexr00]

Thank you for the PR and for your patience with my slow review!