RFR: 8345039: IGV: save user-defined node colors to XML

[Tobias Holenstein]

This PR depends on https://github.com/openjdk/jdk/pull/22402 

[JDK-8343535](https://bugs.openjdk.org/browse/JDK-8343535) introduced the possibility to give user-defined colors to a node.
The colors are lost when the user goes to the next graph or when IGV is closed. Save the colors as a property of the graph to the XML to make it more permanent. This requires that the user can also remove the colors again.

<img width="474" alt="color" src="https://github.com/user-attachments/assets/c994f93c-7db1-40e6-9963-2f5417cec77f">

### Whats new
- Now colors are saved with the XML as well
- Colors are kept when changing to a different graph
- The user can remove the color again: This uses the color from the filter or WHITE otherwise

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Depends on: https://git.openjdk.org/jdk/pull/22402

Commit messages:
 - JDK-8345039: IGV: save user-defined node colors to XML

Changes: https://git.openjdk.org/jdk/pull/22440/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22440&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8345039
  Stats: 60 lines in 5 files changed: 54 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/22440.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22440/head:pull/22440

PR: https://git.openjdk.org/jdk/pull/22440