Remade ItemRegister class and made FileHandling for ItemRegister-objects with tests

ItemRegister now take in objects that extends Items (which means that the objects can only be Income or Expense). Made FileHandling class that has a method that can write an ItemRegister to a file. And a method that can read ItemRegister that contains Income from a file, and ItemRegister that contain Expense. Also made tests for FileHandling

changed milestone to %Code Minimal Viable Product (MVP)

added Backend label

requested review from @hssoreid

assigned to @andreksv

closed