The connect_proxy signal is emitted after connecting a proxy to an action in the group.
Note that the proxy may have been connected to a different action before.
This is intended for simple customizations for which a custom action class would be too clumsy, e.g. showing tooltips for menuitems in the statusbar.
UIManager proxies the signal and provides global notification just before any action is connected to a proxy, which is probably more convenient to use.