6923: Retain treeview stack trace while switching between JFRs

[guruhb]

Persist Treeviewer expanded items during main view (JfrEditor) switch. This behavior will help comparing two jfr recording.

Solution :

1. StackTraceView is a Viewpart which lists the stacktraces of JfrEditor (EditorPart). Switching between JfrEditor or selecting other context (Event Browser, Filtered events, etc..) makes an update in stacktraceview by providing "Items" to list. This Lists are then presented to StackTraceModel with reducedTree (i.e only the Top frame is expanded) and rest of them are listed with its root.
   With this fix, Persisting the ExpandedElements based on Items and then setting back during Editor Switch.
2. StackTraceFrame requires hashCode and equals in order to Distinguish between old and new frames. Or persisting the StackTraceModel will be bit more expensive then set and restoring the Expanded elements.

---

Progress

• Commit message must refer to an issue
• Change must be properly reviewed

Issue

• JMC-6923: Retain treeview stack trace while switching between JFRs

Reviewers

• @bric3 (no known github.com user name / role)
• Alex Macdonald (@aptmac - Committer)

Reviewing

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[openjdk]

@guruhb This change now passes all automated pre-integration checks.

ℹ️ This project also has non-automated pre-integration requirements. Please see the file CONTRIBUTING.md for details.

After integration, the commit message for the final commit will be:

6923: Retain treeview stack trace while switching between JFRs

Reviewed-by: aptmac

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[guruhb]

/integrate

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@guruhb Pushed as commit df466e7.

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