[Core] Improve Kernel events to allow boot level events later

Kernel verbosity is level `VERBOSITY_VERY_VERBOSE`:
http://symfony.com/doc/current/components/console/introduction.html#verbosity-levels
We will spare `VERBOSITY_DEBUG` to boot level events that will explain
in more detail what is happening at each boot level.
[Core] Create boot level events

This creates a structure that will allow the boot levels logic to be outside
the Kernel. Furthermore it allows extensions by simply subscribing to these
events and make custom logic to accept that a certain boot level was reached.

If later we allow custom bootstrap levels, this will support it a lot easier.