add new events to core

[Schlaefer]

I want to introduce a few new events:

• after core init: access the response (e.g. change status code from 404 to 200 to serve different content on 404)
• after 404: detect when a 404 happened
• after resolve page: detect which page was requested and which page is actually served as main page

What above offers is possible at the moment by parsing and comparing file-paths and/or urls, derive it from other events, sending a complete new response etc.

But imho those events would make life for plugins a lot easier because they are more explicit.