Add config option for the notification timeout duration

misc/toxic.conf.example

@@ -53,6 +53,9 @@ ui = {
   // maximum lines for chat window history
   history_size=700;
 
+  // time in milliseconds to display a notification
+  notification_timeout=3000;
+
   // Indicator for display when someone connects or joins a group.
   line_join="-->";