RFR: 8263322: Deadlock when calling Application.launch in the FXThread after Platform.startup

[Florian Kirmaier]

Fixing deadlock when calling Application.launch in the FXThread after Platform.startup

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Commit messages:
 - JDK-8263322

Changes: https://git.openjdk.java.net/jfx/pull/421/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=421&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8263322
  Stats: 85 lines in 2 files changed: 84 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jfx/pull/421.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/421/head:pull/421

PR: https://git.openjdk.java.net/jfx/pull/421