cmd/trace: remove unrelated arrows in task-oriented traceview

Also grey out instants that represent events occurred outside the
task's span. Furthermore, if the unrelated instants represent user
annotation events but not for the task of the interest, skip rendering
completely.

This helps users to focus on the task-related events better.

UI screen shot:
https://gist.github.com/hyangah/1df5d2c8f429fd933c481e9636b89b55#file-golang-org_cl_99035

Change-Id: I2b5aef41584c827f8c1e915d0d8e5c95fe2b4b65
Reviewed-on: https://go-review.googlesource.com/99035


Run-TryBot: Hyang-Ah Hana Kim <hyangah@gmail.com>
Run-TryBot: Heschi Kreinick <heschi@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>