Uses slotting to pass the created comment thread elements to gr-diff.
In the initial rendering phase, the thread elements are redistributed to
the gr-diff-builder, who filters and attaches them as before.
After that, added comments are listened for by gr-diff, and attached to
the appropriate thread group.
With this change, gr-diff and descendants no longer depend on
gr-diff-comment-thread and decendants. However, they still depend on the
specifics of the Gerrit comment model (for computing lines of interest
and for the ranged comment layer), which is a dependency we will cut
next.
Change-Id: Ifdb5da86d6bf6160efba9bc59dd347b1c5e55e85
8c8ce2daba