network-flow(donut): give the component library page its controls

The page drew the same chart three times, once per hardcoded box size, and
that is all it could say: nothing on it reached the legend mode, the
aggregated remainder or the comparison columns, so the states worth looking
at were the ones no reader could get to.

One preview with a properties panel says more with less. Width and height
become fields, which covers the three boxes and everything between them -
including the tightest tile a dashboard allows, where the interesting part is
what the legend gives up. The rest of the panel reaches the states the chart
can be in: either legend, a caption, how many categories are ranked, whether
the remainder is appended, and whether history is delivered. The previous
values are made up per category rather than by one factor, so growth, decline
and growth out of nothing are all on screen at once.

CMK-37876

Change-Id: I65b63965ca16cc650f3811265ae1faa243dbbd79