Document Event API

[waltster]

Add documentation for the Event API. This should include:

• [ ] Basic description and flowchart of event handling
• [ ] Writing an event handler class
  • [ ] Event annotations
  • [ ] Methods to handle event
• [ ] Installing event handler
• [ ] Uninstalling event handler
• [ ] Cleanup of handlers on plugin-uninstall
• [ ] Event priorities
• [ ] Canceling events