Merge of Sophia's, Jakub's and Adam's work into milestone-02

See merge request xkridl/pv168-project!16

Note: Now is used Observer pattern and the resulting architecture is more intuitive.

Create business logics class (and its corresponding interface). Make AddCategoryAction and AddTransactionAction work through it.

Introduce wrappers, i.e. classes for trivial management of dialogs/entity models/selection model inside business logics.

Note: Abstract Factory design pattern is used here.

Refactoring: Create generic EntityTableModel, make CategoryTableModel and TransactionTableModel inherit from it.

Note: This step hugely removes duplicity from two latter table models.