RFR: 8262462: IGV: cannot remove specific groups imported via network

[Roberto Castañeda Lozano]

This change connects groups to their `GraphDocument` parents when the groups are imported via network, making it possible to remove the imported groups later. Before the change, this connection was made only when groups are imported from a file:

https://github.com/openjdk/jdk/blob/1d66a155c711906fbb5e8e976fd6585ef491ea68/src/utils/IdealGraphVisualizer/Coordinator/src/main/java/com/sun/hotspot/igv/coordinator/actions/ImportAction.java#L147

Tested manually on JDK 8, 11, and 15 (latest version currently supported by IGV) by following the steps in the [bug report](https://bugs.openjdk.java.net/browse/JDK-8262462).

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Commit messages:
 - Set group's parent when a group is added to the graph document

Changes: https://git.openjdk.java.net/jdk/pull/3515/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3515&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8262462
  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3515.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3515/head:pull/3515

PR: https://git.openjdk.java.net/jdk/pull/3515