RFR: 8369306: Implement invokeAndWait and finishTerminating on headless platform

[Jose Pereda]

This PR implements `invokeAndWait` and `finishTerminating` for the headless platform and includes two system tests, one for each, that fail before this PR, and pass after it.

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Commit messages:
 - Implement invokeAndWait and finishTerminating on headless platform, including tests

Changes: https://git.openjdk.org/jfx/pull/1934/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1934&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8369306
  Stats: 160 lines in 4 files changed: 159 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jfx/pull/1934.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1934/head:pull/1934

PR: https://git.openjdk.org/jfx/pull/1934