Adds main method.

Adds main method to Core class. Main method creates new CalculatorGUI object.

Signed-off-by: Pyry Nikunen <pyry.nikunen@cs.tamk.fi>
Adds UserInterface interface.

UserInterface will contain UI methods.

Signed-off-by: Pyry Nikunen <pyry.nikunen@cs.tamk.fi>
Adds imports and implement.

Adds import for UserInterface and makes CalculatorGUI implement UserInterface.

Signed-off-by: Pyry Nikunen <pyry.nikunen@cs.tamk.fi>