move ComboBoxDisplay to scenery-phet, https://github.com/phetsims/scenery-phet/issues/449
move ComboBoxDisplay to scenery-phet, https://github.com/phetsims/scenery-phet/issues/449

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

phetsims/scenery-phet,phetsims/scenery-phet,phetsims/scenery-phet