RFR: 6923: Retain treeview stack trace while switching between JFRs

[Guru Hb]

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.

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Commit messages:
 - renaming field
 - 6923: Retain treeview stack trace while switching between JFRs

Changes: https://git.openjdk.java.net/jmc/pull/259/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jmc&pr=259&range=00
  Issue: https://bugs.openjdk.java.net/browse/JMC-6923
  Stats: 56 lines in 2 files changed: 54 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jmc/pull/259.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/259/head:pull/259

PR: https://git.openjdk.java.net/jmc/pull/259