If an NmtEditorGrid row has a widget, but no label, then we make the
widget span both the label and widget columns. But previously we
weren't doing the same for rows with labels but no widgets. (In fact,
we didn't even allow rows with no widgets; label-only rows had to
specify dummy widgets.)
Fix it so that labels will span into an empty widget column. (This
ensures that a long section name won't force the entire grid to have
an overwide label column).
Also, in both the "no label" and "no widget" cases, still show the
"extra" column if it's present.
Rename NmtPageDevice to NmtEditorPageDevice, and rename NmtPageGrid to
NmtEditorGrid.
Now all types with names starting with "NmtPage" are instantiable page
types, not abstract classes or low-level widgets.
