Added functionality of adding new opinions to databse by GUI

* What did I?
User has possibility to add new opinion to database by click 'Save!'
button

* How did I?
- add model.database package
- define operations of adding new opinions by OpinionFileDatabase class

* Important
- implemented operation of updating refers only to text of opinion
  (you cannot change for example type of opinion)
- operations of getting, deleting opinions are already unsupported