When the mouse is over or a keyboard driven menu is open, avoid
unexpected expiration to mess with the current notifications.
This commit also improve the `suspended` behavior to correctly
emit some signals.
The current API is non-compliant with the 1.0 spec and cannot represent
the v1.2 spec at all. The pair of name and callback fails to represent
the explicit ordering and cannot support the icons cleanly.
Plus to support the keyboard navigation use case, the notification
action need to be able to get some sort of focus state. Having an
object makes this easy.