Create form

it's a base abstract class to provide an ability of using polimorphism.
each derived class represents one type of form.
this class contains 5 virtual methods: 1 public - clear() (it's used to put figure in initial condition) and 4 private - functions-helpers - to help figures to rotate (it's possible to reduce amount of these helpers to two, but I decided it would be harder to understand and improve the code).