RFR: 8214158: Implement HostServices.showDocument on macOS without calling AWT

[Michael Paus]

Fixes the issue by using the "open" command as discussed before.
See: https://bugs.openjdk.java.net/browse/JDK-8214158

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Commit messages:
 - Remove dependency on java.awt.Desktop

Changes: https://git.openjdk.java.net/jfx/pull/634/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=634&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8214158
  Stats: 3 lines in 1 file changed: 1 ins; 1 del; 1 mod
  Patch: https://git.openjdk.java.net/jfx/pull/634.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/634/head:pull/634

PR: https://git.openjdk.java.net/jfx/pull/634