Add config option to set data file auto-save frequency

misc/toxic.conf.example

@@ -47,6 +47,9 @@ ui = {
   // How often in days to update the DHT nodes list. (0 to disable updates)
   nodeslist_update_freq=7;
 
+  // How often in seconds to auto-save the Tox data file. (0 to disable periodic auto-saves)
+  autosave_freq=600;
+
   // maximum lines for chat window history
   history_size=700;