RFR: JDK-8251241 fixing mac minimize

[Florian Kirmaier]

ticket: https://bugs.openjdk.java.net/browse/JDK-8251241

This small change fixing the minimize for mac.
Changes are property registered.
Calling setIconize(true) no longer resets to false.

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Commit messages:
 - JDK-8251241

Changes: https://git.openjdk.java.net/jfx/pull/280/files
 Webrev: https://webrevs.openjdk.java.net/jfx/280/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8251241
  Stats: 8 lines in 1 file changed: 4 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/jfx/pull/280.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/280/head:pull/280

PR: https://git.openjdk.java.net/jfx/pull/280