RFR: 8263322: Deadlock when calling Application.launch in the FXThread after Platform.startup [v6]

[Florian Kirmaier]

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

Florian Kirmaier has updated the pull request incrementally with two additional commits since the last revision:

 - JDK-8263322
   Small changes based on codereview
 - JDK-8263322
   Some small cleanups based on codereview

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Changes:
  - all: https://git.openjdk.java.net/jfx/pull/421/files
  - new: https://git.openjdk.java.net/jfx/pull/421/files/145ae592..92e0fca6

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jfx&pr=421&range=05
 - incr: https://webrevs.openjdk.java.net/?repo=jfx&pr=421&range=04-05

  Stats: 7 lines in 1 file changed: 3 ins; 4 del; 0 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