RFR: 8231372: Correctly terminate secondary event loop in JFXPanel.setScene()

[mruzicka]

Secondary event loop introduced as a means of synchronization with the JavaFX Application thread in [1] never terminates as the SecondaryLoop.exit() call is not reached because the thread is blocked in the SecondaryLoop.enter() call.
This patch fixes the problem by submitting the UI work (including the call to the SecondaryLoop.exit() method) before entering the secondary loop.

[1] https://github.com/openjdk/jfx/commit/7cf2dfa0b3c5bfd0f1a2de36d46b62f7e9e256c4

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Commits:
 - 9483ccde: 8231372: Correctly terminate secondary event loop in JFXPanel.setScene()

Changes: https://git.openjdk.java.net/jfx/pull/16/files
 Webrev: https://webrevs.openjdk.java.net/jfx/16/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8231372
  Stats: 6 lines in 1 file changed: 1 ins; 2 del; 3 mod
  Patch: https://git.openjdk.java.net/jfx/pull/16.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/16/head:pull/16

PR: https://git.openjdk.java.net/jfx/pull/16