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
- `negativeAction`: callback function for the negative button