Allow to specify a custom similarity algorithm for notifications

Right now the similarity algorithm for Notifications is hardcoded and cannot be changed. This algorithm is responsible of grouping several events in a CompositeEvent that will be displayed in the notifications.
Grouping together events that should not be could be felt as really wrong for the users, not only because we display them together, but also because some actions such as "Mark events as read" are actually using the composite events.

So it looks pretty important to allow extension developer to define some custom grouping strategies for specific kind of events.