rename ComboBoxItemNode to ItemNode, https://github.com/phetsims/sun/issues/429
rename ComboBoxItemNode to ItemNode, https://github.com/phetsims/sun/issues/429

Signed-off-by: Chris Malley <74b2712074b149d140079dbd7e875955d192e617@pixelzoom.com>

phetsims/sun,phetsims/sun,phetsims/sun