IGV: Graph changes when deleting a graph in the same group with smaller index

The `RangeSliderModel` in IGV selects which graph to display from the `Outline`. `RangeSliderModel` just saves the group and the index of the graph within the group. If the user deletes a graph in the same group but with a smaller index as a showing graph, `RangeSliderModel` selects the wrong graph because the index of the graph has changed. This is not expected