Custom filled dialog example: handle back button to close

On Android, it should prefer to close the dialog rather than get stuck
or exit the application.  (It was already possible to close the dialog
by tapping outside, though.)  The back button can also be used on
other platforms which have them (e.g. multimedia keyboards on desktop).

Change-Id: I19d2ad62ff370fa467d80d51d73cb95fd5830bed
Reviewed-by: J-P Nurmi <jpnurmi@theqtcompany.com>