Add in HorizontalLegend.

Lists entries side-by-side in the available width, after which the
entries wrap onto the next row. If an entry is too wide it is
truncated. If there are too many entries some rows will not be shown.

Close #755.
The name desiredWidth seems a bit of a misnomer as this is read as "You get the width you always wanted," in which case, why would it want more?