Inspector: fix a bug that prevented the open probe group data frame from being
closed automatically after all required probe values have been added.
Inspector: re-implement placing vertical rules in probe group data grids when
the main frame changes.

Separators are now implemented as a flag on the preceding data frame, which
causes a vertical rule to be rendered by the DataGrid subclass.

This is possible due to the simplifying assumption that vertical rules can only
appear after a frame (not before the first frame), and that multiple adjacent
reloads are represented by a single vertical rule.