RFR: 8206430: Use consistent pattern for startup in FX system tests

[Andy Goryachev]

1. Introduced the following utility methods:
- Util.launch
- Util.shutdown
- Util.waitForLatch
2. Fixed the out-of order calls to Stage.hide() and Platform.exit() in many tests' shutdowns.
3. Replaced local waitForLatch copies with Util.waitForLatch

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Commit messages:
 - 8206430: wait for latch
 - 8206430: exit test
 - 8206430: wait for latch
 - 8206430: t..w
 - 8206430: r..s
 - 8206430: h..p
 - 8206430: util.shutdown
 - 8206430: c..f
 - 8206430: util.launch

Changes: https://git.openjdk.org/jfx/pull/950/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=950&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8206430
  Stats: 2255 lines in 91 files changed: 603 ins; 1094 del; 558 mod
  Patch: https://git.openjdk.org/jfx/pull/950.diff
  Fetch: git fetch https://git.openjdk.org/jfx pull/950/head:pull/950

PR: https://git.openjdk.org/jfx/pull/950