Fix the vertex cell-grid source.

+ The shape function must have a function space named "constant".
+ Also, the "point" array-group is renamed "points" to match the
  group name for the point coordinates so that HGRAD values are
  shown in the array editor.
+ Finally, the centers of all sides (not just the edges) are
  glyphed when rendering glyphs to better illustrate the vector fields
  in the CellGridSource application.
+ Make Ctrl+Q a shortcut to quit the example CellGridSource app.