Fixed a scrollbar bug

Fixed a bug where a large dialog would show a scrollbar, but when clicking the scrollbar the popup would disappear. To do this I had to move the 'ngdialog-overlay' div out of the 'ngdialog' div. To this end I've added a new 'ngdialog-wrapper' div that contains everything.

To make it all work several changes to the style sheets were needed making it look like a really big change, but not a whole lot of code actually changed.