Fix alignment of rows when text width is variable

Fix alignment of rows when text width is variable

When several lines of text are in the same variable-width tabular
cell, it is not possible to align properly the rows until the cell
width is known.

Therefore a parameter is added to redoParagraph to skip this
computation, so that it can be done later in TextMetrics::metrics.
Other calls to redoParagraph in the code are not affected. It is not
clear at this point whether they may create artefacts.

computeRowMetrics has been renamed to setRowAlignment to better
reflect its use.

Fixes bug #11447.