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.
pls refactor `packRows` to be a pure helper function.
we might discover that this actually belongs in a Util, since it is fundamental layout logic.