Make MachineInfo a singleton class

Because on windows MachineInfo take a little time to init, we make it to
a singleton.
MachineInfo is build the first time in main.cpp.
Now all other places where we use MachineInfo (aboutqetdialog and
configdialog) gui don't hang anymore in waiting to MachineInfo finish to
build.

sources/main.cpp

@@ -208,8 +208,7 @@ int main(int argc, char **argv)
         qInfo("Start-up");
             // delete old log files of max 7 days old.
         delete_old_log_files(7);
-		MachineInfo ma;
-        ma.send_info_to_debug();
+		MachineInfo::instance()->send_info_to_debug();
     });
 	return app.exec();
 }