Refactor the Cache Building process.

* extract class
* split up in different methods
* clean up comments
Remove unused use statements.
Add missing comment.
Refactor locale cache building to an external class.
Use dependency injection.

The database is a dependency for the CacheBuilder class. Inject it
trough constructor injection, and not with the static BackendModel::get
method.
Breaks between methods.
Remove unneeded use statement.
Fix variable name.
Refactor towards more testable code.
Merge branch 'master' into refactoring

Conflicts:
	src/Backend/Modules/Locale/Engine/Model.php
	src/Backend/Modules/Pages/Engine/Model.php