duplicated code for Gravity combo box

[pixelzoom]

Noted while working on https://github.com/phetsims/sun/issues/428#issuecomment-451622311.

Code that creates the items for the Gravity combo box is duplicated, see GravityAccordionBox and GravityAndDampingControlNode:

    //  Add gravity info for various planets
    var bodyListItems = [];
    var bodies = Body.BODIES;
    Body.BODIES.forEach( function( body ) {
      var bodyLabel = new Text( body.title, {
        font: MassesAndSpringsConstants.LABEL_FONT,
        maxWidth: MAX_WIDTH,
        tandem: tandem.createTandem( 'bodyLabel' )
      } );
      bodyLabel.localBounds = bodyLabel.localBounds.withX( 107 );
      bodyListItems.push( ComboBox.createItem( bodyLabel, body ) );
    } );

So I had to do the same work twice for https://github.com/phetsims/sun/issues/428. Highly recommended to factor out GravityComboBox and use it in both sims.

[Denz1994]

GravityComboBox.js has been created to remove the code duplication. Any differences in the combo boxes prior to the refactor have been added to the options at the repo-specific call site. Also, some variables have had their exposure made private due to the above changes. Closing.