RFR: 8214069: Use freedesktop's xdg-open to show document on UNIX systems

[Tristan Deloche]

Hello,

Please review the fix for The Bug Synopsis:
https://bugs.openjdk.java.net/browse/JDK-8214069
https://github.com/javafxports/openjdk-jfx/pull/293

Best regards,
Tristan Deloche