We use dconf-editor to update the system-tray whitelist.
You can easily install it with apt-get.
sudo apt-get install dconf-tools
You can either type dconf-editor on terminal or click on dash home and search for dconf-editor.
After opening dconf editor, Navigate to Desktop > Unity > Panel there you can find systray-whitelist.
Double click on the value and update to ['all'] or add your application name to the list.
Eg. Initially the value will be
['JavaEmbeddedFrame', 'Wine', 'scp-dbus-service', 'Update-notifier', 'Jupiter']
After adding Xchat and Zeal, it become
['JavaEmbeddedFrame', 'Wine', 'scp-dbus-service', 'Update-notifier', 'Jupiter', 'Xchat', 'Zeal']
or if you are adding all,
You are done. Now just logout and login for changes to take effect. After that When you minimise Xchat or Zeal you can find their icon over system try.