Add Dialog.closable property, to hide the [x] icon and disable ESC as a way to close the dialog.  Also removing aria-describedby property, as this is handled implicitly by _WidgetBase.  Fixes #8345 !strict.
Add Dialog.closable property, to hide the [x] icon and disable ESC as a way to close the dialog.  Also removing aria-describedby property, as this is handled implicitly by _WidgetBase.  Fixes #8345 !strict.

git-svn-id: 674a2d6b1e32e53f607eb824547723f32280967e@30966 560b804f-0ae3-0310-86f3-f6aa0a117693

harbalk/delite,kitsonk/delite,kitsonk/delite,harbalk/delite,Sam-Cummins/delite,Sam-Cummins/delite