* New event that lets plugins trigger notifications in the admin area

They have to be triggered before assigning the notifications to the view and there is no other way to determine when a plugin is supposed to trigger notifications in the admin. Ideally, in the future we would even move the notifications above to CoreAdminHome or somewhere else eventually.

First thought of using `Controller.addAdminNotifications` as we often use this wording but then we would need to rather do `Piwik::postEvent('Controller.triggerAdminNotifications', &$notifications);` as otherwise nothing can be added.

Plugins could actually also use this event to cancel notifications where in the past it was needed to hide them via CSS. 

Happy about any other naming suggestions but want to keep it "specific" to notifications and not something to general like `postEvent('Controller.renderAdminView, $view')`.

* document new event