Implement openURL on Linux

Implement openURL using xdg-open, the freedesktop.org command to
open a resource in the default browser or viewer.

Closes surge-synthesizer#638 menu manual on linux broken

src/linux/UserInteractionsLinux.cpp

@@ -1,6 +1,7 @@
 #include "UserInteractions.h"
 #include <iostream>
 #include <iomanip>
+#include <strstream>
 
 namespace Surge
 {
@@ -31,6 +32,9 @@ namespace Surge
 
         void openURL(const std::string &url)
         {
+           std::ostringstream oss;
+           oss << "xdg-open " << url;
+           system (oss.str().c_str());
         }
     };
 };