The goal of this commit is to make painting faster by reducing the
number of strings to paint. To this end, it is necessary to include
spaces in row elements.
Also importantly, this commit should fix existing problems with line
breaking in chinese text.
* TextMetrics::breakRow: do not do anything special for word separators.
* Row::add: when adding a character to a row element, keep the string
width updated. If need be, it is possible to tweak this by updating
every 10 characters, for example.
* GuiFontMetrics::breakAt (new): use QTextLayout to break text either
at word boundary or at an arbitrary width.
* Row::Element::breakAt: use the above method.
* Row::shortenIfNeeded: simplify now that because there is no need for
handling separator elements. This will be taken care of by the
improved breakAt.