Adding comment features?

[kang-sw]

It'd be nice to comment feature is implemented. In Unreal Engine's blueprint editor, there are two comment types:

• Comment panel => Visually groups multiple node with colored panel, has title and color

• Node comment bubble => Attach comments onto individual nodes.

I think it's just okay to putting comment data as 'metadata' of Snarl container, and giving users to opt-out metadata when required. (e.g. Snarl::strip_metadata(&mut self)

And if #7 (selecting multiple node at once) is once implemented, it'd be possible that creating comment panel with bound size
automatically calculated from selected nodes.

[zakarumych]

SnarlViewer may output on-hover pop-up for elements, so user may use it for 2nd style comments.

There's is a plan for groups which will be optional part of Snarl.
SnarlViewer will be responsible to populate it with content which again can contain comments.