Fix barely noticeable flicker when dragging an item beyond window boundaries
Fix barely noticeable flicker when dragging an item beyond window boundaries

Also,

  * Implement vertical boundaries check.

  * Allow margin between draggable item and window boundaries to be
    configurable.

dossier/sortingdesk,dossier/sortingdesk,dossier/sortingdesk,dossier/sortingdesk,dossier/sortingdesk