Slider: API for setting options is clunky

[tig]

Currently:

    /// <summary>Causes the specified option to be set and be focused.</summary>
    public bool SetOption (int optionIndex)
    /// <summary>Causes the specified option to be un-set and be focused.</summary>
    public bool UnSetOption (int optionIndex)

The problem is these APIs want to change which option is focused.

As part of #3705 I want to use Slider in ArrangementEditor and this is causing me fits.

So I will fix this there...