x/tools/cmd/heapview: add a sidebar to hold navigation

This change also puts more structure into the viewer.
Adds an enum for events that we'll issue and a few more elements
to organize things.

Change-Id: I39c7c53422779348ca05f051c6b0b07d22ad6a00
Reviewed-on: https://go-review.googlesource.com/26656
Reviewed-by: Alan Donovan <adonovan@google.com>
4 files changed