Adds API 1.3 `ui.notify(level, title, body, actions) -> result<_, error>`: up to three buttons, each running one of the plugin's declared commands with an optional argument in the balloon's window and dismissing the balloon. An invalid action or an undeclared command fails the call and shows nothing. The plugin `Notification` event carries the actions; SDK `host::notify_with_actions` and `NotificationAction`. Closes#1955.
Co-authored-by: arnaud-ritti <[email protected]>
