Add a notification system for the manager backend.

To create an notification just emit an alert event with an object to
configure the alert.
This options object includes following options:
- type: string which specifies the type of the alert (available are:
  alert-success, alert-error, alert-info or it can be left empty)
- heading`: string which spefifies a header for the alert
- text: string which specifies the message of the alert
- modal: boolean which spefifies if the alert should be modal
  (modal alerts have buttons for a positive and a negative action,
  non-modal alerts have a dismiss button)
- timeout: number which specifies after what amount of time non-modal
  diaglogs should disappear (in seconds, default is 60)
- positiveAction: callback function for the positive button (only for
  modal alerts)
- negativeAction: callback function for the negative button (only for
  modal alerts)