Datagrids can now be serialized to JSON, providing information on active

and available columns, pagination, sorting information, rows, and each

cell within. The cells contain both rendered HTML (the <td> and

everything inside) and a JSON value for processing.
This can be used to provide datagrid functionality in other contexts.

The primary purpose at this time is to enable API access to datagrids.

The information can be returned as-is or further processed to provide

a more integrated response into an API response format.
Columns can provide an explicit JSON value alongside rendered HTML. Both

now default to making use of a new get_raw_object_value(), which can

provide a suitable value from an object. This is intended for column

implementations to override and use as necessary, and to simplify common

code used for render_data() and to_json().

Unit tests pass.