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.

Test shifted baseline.

Test mixed line height with varied alignment.

Add content rect to Fragment.

Test preservation of lines with boxes.

Test boxes with no text or only a space.

Allow trimmed whitespace to generate empty fragments.

BREAKING: Output may now contain more fragments with zero glyphs,
representing runs of whitespace that was trimmed away. These runs may
also start a box at the end of a line, instead of the beginning of the
next line.

BREAKING: Trimmed whitespace now affects line height.

Test with AlignStart by default.

Test centred paragraph.

Allow paragraphRect without point 0,0.

Test right-aligned paragraph.

Test handling of empty lines.

Test mixed line height.

Improve BiDi handling of numbers and neutrals.

Test with only directionally neutral characters.

Test mirrored neutral characters.

Add tests for mixed text direction.

Describe box edges with extensible type.

Test interaction between boxes and mixed script.

Test line breaking in boxes.