RFR: 6916: JOverflow treemap listener depending on internal SWT class

[Marcus Hirt]

Also removing unused constant.

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Commit messages:
 - 6916: JOverflow treemap listener depending on internal SWT class

Changes: https://git.openjdk.java.net/jmc/pull/116/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jmc&pr=116&range=00
  Issue: https://bugs.openjdk.java.net/browse/JMC-6916
  Stats: 8 lines in 2 files changed: 2 ins; 4 del; 2 mod
  Patch: https://git.openjdk.java.net/jmc/pull/116.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/116/head:pull/116

PR: https://git.openjdk.java.net/jmc/pull/116