RFR: 8345039: IGV: save user-defined node colors to XML [v2]

[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

Tobias Holenstein has updated the pull request incrementally with two additional commits since the last revision:

 - patch2: fix botton on linux
 - patch1

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/22440/files
  - new: https://git.openjdk.org/jdk/pull/22440/files/f0de94b7..e58c6171

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=22440&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=22440&range=00-01

  Stats: 20 lines in 3 files changed: 16 ins; 0 del; 4 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