Notification displayers

We need a generic way to customly display interactions with or modifications of entries inside the wiki. Currently this can be applied to Activity Stream and to the Watchlist emails.

Something similar to what we currently have for document/class sheet binding that can be attached to the class page:

• name: the name used to reference an item of this class
• icon: the icon used to display an item of this class
• creationDisplayer: code used to display a new item. This could be a reference instead.
• modificationDisplayer: code used to display a modified item. This could be a reference instead.

We could call it something like ClassEntryDescriptorClass and it could be reused wherever we need to list activity on entries of the described class.

We might also need a script service to properly use the displayers (set some variables in the velocity context before rendering them).