Update GroupBox to support contentItem

- Removed contentWidth and contentHeight since it is not needed
- Exposed contentItem so that it can be designable.
- Respect the implicit width or height of the contentItem if it is the
only item of the GroupBox. Otherwise childrenRect is used.
- Modified the style so that the style takes care of only the
padding size and not the entire size of the group box.
- Fixed flat and checkable appearance for custom style

Change-Id: I2c7ec4cb7d5e6f96863847e8d8d6d6f52428364e
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>