RFR: JDK-8291823: IGV: Fix "Save selected groups" [v3]

[Tobias Holenstein]

On Thu, 4 Aug 2022 08:36:10 GMT, Roberto Castañeda Lozano <rcastanedalo at openjdk.org> wrote:

> Thanks for working on this @tobiasholenstein!
> 
> I tested the changeset manually and found that, after selecting all groups listed in the Outline window and clicking on `Remove selected graphs and groups`, we end up with an empty outline where the `Save selected groups to XML file...` and `Remove selected graphs and groups` buttons are still enabled:
> 
> ![empty-outline](https://user-images.githubusercontent.com/8792647/182801237-2a8d76f3-34dd-4714-a6f9-c5164ad459c7.png)
> 
> Nit: to ease reviewing (and backporting, etc.), try to minimize the number of empty lines added/removed that are not directly related to the changeset.

Good catch! The problem here was that all the `FolderNodes` are in an invisible root `FolderNode` that gets selected when all its children are removed. I fixed it by checking that the selected `FolderNode` is not the root.

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PR: https://git.openjdk.org/jdk/pull/9730