Fix Scrollbar on 2nd+ Dialog Open/Close

Fix scrollbar on 2nd+ dialog open/close
When two or more dialogs are open the `data-last-overflow` attribute is
set to `hidden` as the first dialog sets body to `hidden`

- Apply dialog count check

open, beforeClose, ui-dialog, dialog, body, overflow, hidden
Merge pull request #991 from mkly/fix_body_scrollbar_disappear

Fix Scrollbar on 2nd+ Dialog Open/Close