Use xdg-open to open URLs.

On GNU/Linux (or similar, non-macOS, non-Windows platforms), when
attempting to open a URL, try using the `xdg-open` command first, before
any other possible browser. This would ensure that the link is open
using the user's preferred browser (assuming a FreeDesktop.org-compliant
desktop environment).

closes #1263

protege-editor-core/src/main/java/org/protege/editor/core/ui/util/NativeBrowserLauncher.java

@@ -34,7 +34,7 @@ else if (OSUtils.isWindows()){
             }
             else { //assume Unix or Linux
                 String[] browsers = {
-                        "firefox", "opera", "konqueror", "epiphany", "mozilla", "netscape" };
+                        "xdg-open", "firefox", "opera", "konqueror", "epiphany", "mozilla", "netscape" };
                 String browser = null;
                 for (int count = 0; count < browsers.length && browser == null; count++)
                     if (Runtime.getRuntime().exec(