ui: show fluid results in the viewport with the part, legend and controls

The window built a deformed copy of the model for every results plot,
which drops the geometry surfaces, so after a fluid solve the bracket
vanished and nothing else was drawn.

A fluid step's plot keeps the part as it is and draws the engine's flow
display through it (the same scene as the headless pictures): the
field on a slice, the part coloured by the flow beside it and
streamlines. The legend shows the field with its bands and the cells
of its extremes (Max/Min mark and turn the view to them); the results
panel swaps the deformation rows for the slice's normal and position,
the obstacles' colours, the streamlines and a frame slider, all
running the results.* commands. The status line describes the new
commands.

Part of #193.

docs/examples.md

@@ -67,8 +67,10 @@ Reynolds number and rises as it falls); the lattice's mass changes by less than
 
 ![The L-bracket the air flows past, as the sample opens](media/examples/flow-past.png)
 
-The picture shows the bracket only: the viewport does not draw flow results yet (velocity
-slices and streamlines are planned in M12), so the results are numbers for now.
+The picture shows the bracket as the sample opens. After Solve the viewport draws the flow:
+velocity magnitude on a slice through the middle of the box (`results.slice`; pressure with
+`field: "pressure"`), the bracket coloured by the flow beside it (`results.surface`) and
+streamlines seeded upstream of it, coloured by speed (`results.streamlines`).
 
 ## Heated bracket (`heated-bracket`)