Add paragraphLines to Rich ParagraphLayout.

BREAKING: The paragraphLines field of ParagraphLayout must be preserved
for pagination.

BREAKING: The width of a rich text ParagraphLayout will now remain
unchanged after pagination.

BREAKING: Empty space at the top of the first line and at the bottom
of the last line in a ParagraphLayout will now be preserved.

Add content rect to Fragment.

Describe box edges with extensible type.

Handle ancestor boxes in rich layout.

Add ancestor boxes to interface.

Test layout with mixed font sizes.