RFR: 8245282: ButtonBehavior: memory leak on dispose

[Jeanette Winzenburg]

Reason for the memory leak is a listener on control's focusProperty that is not correctly removed on dispose. For
details please see the report.

Added a test method to ButtonSkinTest that failed before and passes after the fix.

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Commit messages:
 - 8245282: ButtonBehavior: memory leak on dispose

Changes: https://git.openjdk.java.net/jfx/pull/226/files
 Webrev: https://webrevs.openjdk.java.net/jfx/226/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8245282
  Stats: 22 lines in 2 files changed: 17 ins; 3 del; 2 mod
  Patch: https://git.openjdk.java.net/jfx/pull/226.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/226/head:pull/226

PR: https://git.openjdk.java.net/jfx/pull/226