Dialogs: buttonless fully-filled dialog example

The default property "contentItem" is normally the parent of the
added content, but you can set the content directly into the property
to avoid having any buttons or margins.

Task-number: QTBUG-38056
Change-Id: Ie0e96969b3e35f079b29a9f3f57eaaf16236d4c1
Reviewed-by: Liang Qi <liang.qi@digia.com>