Fixed #8768: Modal - Screen Flickers on Open and Hide

This pull request is about issue #8768

Screen flickers during animated Modal opening/hiding because of double scrollbars:
 * First scrollbar for body - it is not hidden yet.
 * Second scrollbar for modal - it is shown already.

Solution is:

 1. Kill scrollbar in same moment when the modal is shown. Reanimate scrollbar after hide of the modal.
 2. Prevent body from moving because of greater width without scrollbar.

Actually it is the same as fancyBox.js acts.