[dash-table] Improve dropdown props typing

[Marc-Andre-Rivet]

dropdown_properties, column_static_dropdown, column_conditional_dropdowns are very loosely typed at the moment -- that typing can be improved, amongst other things by using PropTypes.objectOf which defined the structure of the object's properties but allows any property name.

This relates to #156, #143.

[alexcjohnson]

The new structure as of #446 is pretty complete in its typing.