RFR: 8231372: JFXPanel fails to render if setScene called on Swing thread [v2]

[Kevin Rushforth]

> This fix was originally proposed by @mruzicka in PR #16 which was closed several months ago without being integrated. At the time we didn't have a test case that was failing.
> 
> While evaluating a bug that I filed, [JDK-8235843](https://bugs.openjdk.java.net/browse/JDK-8235843), I discovered that that bug was a duplicate of this one. The original proposed fix is correct, although I modified it slightly to add a try / finally so that the secondary event loop will be terminated even if the setScene throws an exception. I also added a unit test. Since the bulk of the fix is from PR #16, I will add the contributor of that PR.
> 
> NOTE to reviewers: I pushed two commits to my branch. The first is exactly the commit created for PR #16 and the second is the unit test along with the fix to the code to add try / finally. As always, they will be squashed into a single commit by Skara.

Kevin Rushforth has updated the pull request incrementally with one additional commit since the last revision:

  Dispose JFrame after each test

-------------

Changes:
  - all: https://git.openjdk.java.net/jfx/pull/362/files
  - new: https://git.openjdk.java.net/jfx/pull/362/files/6039460b..22e3886d

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jfx&pr=362&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jfx&pr=362&range=00-01

  Stats: 13 lines in 1 file changed: 10 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/jfx/pull/362.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/362/head:pull/362

PR: https://git.openjdk.java.net/jfx/pull/362