Use old approach to update selection's 2D positions

[JordanMartinez]

Resolves #579

[JordanMartinez]

The approach I was using before this PR throws an exception:

// Approach #1
Val<Position> start2DPosition = Val.create(() -> {
    IndexRange sel = internalRange.getValue();
    return area.offsetToPosition(sel.getStart(), Forward);
    },
        internalRange.invalidations()
);
Val<Position> end2DPosition = start2DPosition.map(pos2D -> getLength() == 0 ? pos2D : pos2D.offsetBy(getLength(), Backward));

but the original approach that was used before things were extracted doesn't throw that exception:

// Approach #2
internalRange.addListener(obs -> {
    IndexRange sel = internalRange.getValue();
    start2DPosition.setValue(area.offsetToPosition(sel.getStart(), Forward));
});
Val<Position> end2DPosition = start2DPosition.map(pos2D -> getLength() == 0 ? pos2D : pos2D.offsetBy(getLength(), Backward));

I used Approach 1 because I thought it cleaned up the code better than 2. In this PR, I'm following A2 but with a few things cleaned up.