Add content rect to Fragment.

Add ancestor boxes to interface.

Test Rich pagination.

Test Rich layout.

Deprecate the Plain interface.

Add user data to spans.

Add line numbers to laid out fragments.

Qualify Plain interface.

Reformat golden test output.

The new format should be more in line with the overall code style and
should be more legible even when extended in the future.

Prepare for test modules sharing PrettyShow.