We now display the description of the currently selected Basis

Lifted out the DescriptionDiv class, so that Add Existing and Create New
Property both work the same way.  Moved away from using Rx.async(),
because Haoyi has said that he is about to remove that method.

Enhanced RxSelect so that we can combine multiple selects into a single
choice.  Introduced machinery so that you can force only one select to
be set at a time.