- [feature] polymorphic_on now accepts many

- [feature] polymorphic_on now accepts many
new kinds of values:

- standalone expressions that aren't
otherwise mapped
- column_property() objects
- string names of any column_property()
or attribute name of a mapped Column

The docs include an example using
the case() construct, which is likely to be
a common constructed used here.
[ticket:2345] and part of [ticket:2238]