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>