Dialog widget fixes under IE

There were two bugs that prevented dialogs from properly displaying in IE under some conditions:

`Dom#getViewportSize` reported a 0-size if the page was scrolling under IE. So on pages with scrollbars the Dialog was always considered to be out of the page and never rendered.

`DomEvent` considered IE5 not to be IE8- and thus missed events in case of IE8 in quirks mode, preventing for example to close the dialog.