Prevent page from scrolling when `Modal` is open

[bedrich-schindler]

Currently, when Modal is opened, we can scroll page behind the modal. Other libraries like Material UI, Ant design prevent page from scrolling when modal is displayed. We agreed to implement same behavior for Modal component.

[bedrich-schindler]

@adamkudrna I moved this issue into To do. If you think it is not necessary, we can postpone it and remove it from the milestone v1.0