Game boards are now screens in the application
Font and operands are now given by the app

 * Default font name is now controlled globally
 * Randomizing operands is now done globally
 * Numpad is now in a GridLayout
Removed the numpad class trying to improve layout
Centered the equation
Moved the again button to the top right corner
Now starts new game by switching to next screen
Added results screen
Centered numpad and cleaned the code
Merge branch 'screens'