save current progress at refactoring the NestedSet class

extracted the logic from the NestedSet class into separate classes

simplified the usage of the reworked NestedSet class

various bug fixes and minor refactorings

minor doc block fix

reduced code duplication in the NestedSet\Insert() class

refactored the menu items cache helper a bit

refactored the nested set operation even more (eliminated some code duplication)

removd the executeTransactionalQuery() method from the Model, as there is already an method in the DB class

simplified the SQL queries in the Service models a little bit

refactored the Breadcrumb class