RFR: JDK-8290069: IGV: Highlight both graphs of difference in outline

[Tobias Holenstein]

Previously, IGV highlighted only one graph in the outline when a difference graph is selected using the sliders. 
Now, IGV highlights both graphs used to calculate the difference graph when they are in the same group. 

<img width="395" alt="highlight both graphs" src="https://user-images.githubusercontent.com/71546117/178496847-d9a69e59-9292-43eb-bbfd-8e21de3aa809.png">

IGV colors the nodes in a difference graph with yellow/red/green to highlight the changes. This only worked if the difference graph is calculated using the sliders. Now, difference graphs is also coloured when calculated via the context menu "Difference to current graph" in the outline. 

<img width="522" alt="Show colors" src="https://user-images.githubusercontent.com/71546117/178498687-5675e797-70b8-4c0f-91ad-6f35abf8ecb8.png">

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Commit messages:
 - JDK-8290069: IGV: Highlight both graphs of difference in outline

Changes: https://git.openjdk.org/jdk/pull/9468/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9468&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8290069
  Stats: 36 lines in 5 files changed: 27 ins; 0 del; 9 mod
  Patch: https://git.openjdk.org/jdk/pull/9468.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9468/head:pull/9468

PR: https://git.openjdk.org/jdk/pull/9468