feat: support Unix-like OSes

[trombik]

As discussed in 1746, here is the patch.

In general, Platform.isUnix() is preferred, which will not break Linux support.

isX11() was suggested. I did search in the code and the web. (Unix|UNIX) is used in several places in the code and the documentation. It looks to me that UNIX(R) is used when referring to the registered trademark, and Unix when referring to Unix-like and Unix variants. Thus, isUnix() is used in the patch.

Pull request type

• Feature enhancement -> [enhancement]

Which ticket is resolved?

• Title #1746 Support U*ix variants
  • URL https://sourceforge.net/p/omegat/feature-requests/1746

What does this PR change?

• Support *BSD variants.

Other information

The build was successful, and I am using it. xdg-open is working.