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.
------------- 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
