RFR: Enhancements for Shenandoah's generational mode [v2]

[William Kemper]

> During the course of the development of the generational mode for Shenandoah, we have found this tool useful to demonstrate progress and changes in behavior. We have also found this tool useful for troubleshooting performance issues and debugging crashes. There are many changes here, but these are the highlights:
> * The age and affiliation of a region are encoded in the border and shape of the region (respectively).
> * Phases are encoded with different colors for different generations and whether they have degenerated.
> * A mechanism to record and replay session has been added (record feature is implemented in hotspot and is not yet upstream).
> * Popup windows can be opened for additional detail on regions, as well as their history.
> * The legend shows the number of regions in the state described by the legend item.
> * Visualizer can now 'find' VMs running Shenandoah with region sampling enabled.
> Many months ago we broke backward compatibility on our branch, but we have recently restored it so the time seems right for a PR. Thank you for looking and sorry for the massive number of changes.

William Kemper has updated the pull request incrementally with one additional commit since the last revision:

  Graph panel improvements
  
  Add time scale for graph panel and change the placement for different phase

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Changes:
  - all: https://git.openjdk.org/shenandoah-visualizer/pull/1/files
  - new: https://git.openjdk.org/shenandoah-visualizer/pull/1/files/ab6ccb70..414c0b9f

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah-visualizer&pr=1&range=01
 - incr: https://webrevs.openjdk.org/?repo=shenandoah-visualizer&pr=1&range=00-01

  Stats: 197 lines in 3 files changed: 142 ins; 9 del; 46 mod
  Patch: https://git.openjdk.org/shenandoah-visualizer/pull/1.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-visualizer pull/1/head:pull/1

PR: https://git.openjdk.org/shenandoah-visualizer/pull/1