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.

Test centred paragraph.