Add xdg-open as preferred browser

Author: kovzol

src/main/java/wprover/GExpert.java

@@ -3979,7 +3979,7 @@ public static void openURL(String url) {
                     Runtime.getRuntime().exec("rundll32 url.dll,FileProtocolHandler " + url);
                 } else { //assume Unix or Linux
                     String[] browsers = {
-                            "firefox", "opera", "konqueror", "epiphany",
+                            "xdg-open", "firefox", "opera", "konqueror", "epiphany",
                             "mozilla", "netscape"};
                     String browser = null;
                     for (int count = 0; count < browsers.length && browser == null;