rename node to dora-image-view

The repository, the Python package, the entrypoint and the manifest name all
now agree. `plot` was carried over from the upstream opencv-plot node and no
longer described anything here, so the module, the class and the environment
variables follow the node:

  dora_plot/         -> dora_image_view/
  dora_plot/plot.py  -> dora_image_view/view.py
  Plot               -> View
  PLOT_WIDTH/HEIGHT  -> VIEW_WIDTH/HEIGHT
  --plot-width/-height -> --view-width/-height

Breaking for anyone wiring `path: dora-plot` or setting PLOT_WIDTH; nothing is
tagged yet.

examples/source.py

@@ -1,7 +1,7 @@
 """Synthetic source node: this repository's stand-in for a camera and a detector.
 
 Emits an animated frame, the box around the square moving in it, and a caption --
-the three inputs dora-plot draws -- so the example dataflow needs no camera, no
+the three inputs dora-image-view draws -- so the example dataflow needs no camera, no
 detector and no network.
 """
 
@@ -19,7 +19,7 @@ FRAMES = int(os.getenv("SOURCE_FRAMES", "0"))
 
 BOX = 96
 SPEED = 6
-CAPTION = "dora-plot synthetic source"
+CAPTION = "dora-image-view synthetic source"
 
 
 def main() -> None: