Replaced content_viewed event with appropriate events and deprecated
content_viewed event

Level is a reserved word in sql, sorry.

This makes the '*' observers to be executed always first. This change allows us to implement parent event class catching if we ever decide we need it without breaking BC. This change includes some more unit test fixes.

Do this in new validate_data() instead, thanks Fred.

* fix typos (thanks Rajesh)
* move log related data out from event API specification
* change callable key in definition to callback
* use event data in observers instead of cached records if possible
* event level is now a number 1…100
* improved event safety checks
* add event name and description method
* new can_view() method
* improve unit tests to test all callable types
* improved performance