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 layout behavior in this legend is really different. I aim to merge/unify the classes later on, in the same what that `NewStylePlot` is going to replace the old ones.