add variable row height callback

[vegarringdal]

add variable row height callback to datasource so user can set correct height during array set/filter/sort

[vegarringdal]

really bad sample, but just for me to later when I update the docs

this.ds = new DataSource(new Selection('multiple'), {
      rowHeight: 50,
      groupHeight: 25,
      rowHeightCallback: (x:any) => {
        if (x.index % 3 === 0) {
          return 35;
        } else {
          return 50;
        }
      }
    });

[vegarringdal]

This have a bug when dragged down to bottom and you use mousewheel upwards