Comments: Generalize ping handling with an is_ping comment type flag

[adamsilverstein]

Add an is_ping flag to WP_Comment_Type (built-in pingback and trackback set
it). separate_comments() now groups any registered ping type into the pings
bucket, and Walker_Comment renders any ping type with the compact ping markup,
replacing the literal 'pingback' / 'trackback' string checks.

Rebuilt from #54 as
part of a stacked-PR trial.

Trac ticket: https://core.trac.wordpress.org/ticket/35214

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