Came up while helping Minel on her use cases, instead of manually connecting an ipywidget's observe, we could make it simpler by providing a add_ipywidget_handler(<ipywidget_instance>) that links the ipywidget and Selector.

Thoughts:

• should this be bidirectional? I think yes, any caveats (recurusion etc.?)
• LinearSelector is compatible with IntSlider and FloatSlider, FloatLogSlider
• LinearRegionSelector is compatible with IntRangeSlider and FloatRangeSlider
• I don't think it makes sense for RectangleRegionSelector to be connectable to any corresponding ipywidget?

@clewis7 thoughts?