@EventListener is used for handler methods instead of implementing
a visitor interface.

Consistently use singular for packages which contain mixed kinds of
classes. Moved a few classes to packages that better fit them.

Moved PI timer logic to its own service. The implementation for room
owner changes has been removed for now.

* Moved classes from `de.thm.arsnova` root package.
* New util package
* Do not start interface names with `I`. Appended `Impl` to the
  implementing class instead.
* Renamed learning progress classes to score and move them from
  `.domain` to a new `.services.score` package.
* Do not capitalize acronyms in class names:
    * CASLogoutSuccessHandler -> CasLogoutSuccessHandler
    * SocketIO* -> Socketio*
* Renamed `.socket` package to `.websocket`

(Lecturer/Skill)Question has been renamed to Content. Method names have
not been touched yet to ease reviewing the changes.

Events for all learning progress related actions
are generated, like answering a new question.
The event classes are used as a tag to evict
the cache based on the session.

Updated copyright year and adjusted header format to be consistent over
all files and with other ARSnova projects.