RFR: 7806: Add an export option for Stacktraces in collapsed format

[Jean-Philippe Bempel]

Add a new context menu when clicking on stacktrace view to export
selected stacktraces as collapsed format. With this format it's
easy to manipulate/filter stacktraces as text file and generate
a flamegraph with the perl script from the flamegraph repository
(https://github.com/brendangregg/FlameGraph)

The generated content is copied into the clipboard.

<img width="936" alt="Screenshot 2022-07-08 at 15 49 18" src="https://user-images.githubusercontent.com/4610701/178006101-f3ffba80-6332-4673-bc8e-299321bd6979.png">

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Commit messages:
 - Add attribute selection to export
 - 7806: Add an export option for Stacktraces in collapsed format

Changes: https://git.openjdk.org/jmc/pull/409/files
 Webrev: https://webrevs.openjdk.org/?repo=jmc&pr=409&range=00
  Issue: https://bugs.openjdk.org/browse/JMC-7806
  Stats: 181 lines in 5 files changed: 178 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jmc/pull/409.diff
  Fetch: git fetch https://git.openjdk.org/jmc.git pull/409/head:pull/409

PR: https://git.openjdk.org/jmc/pull/409