pspp-sheet-view: Add "special-cells" property to speed drawing many columns.

A "special cell" is a cell that is editable or activatable.  When a row
that contains a special cell is selected, the cursor is drawn around a
single cell; when other rows are selected, the cursor is drawn around the
entire row.

With the default of "detect" (the only mode previously supported), whether
a given row contains a special cell is detected automatically.  This is the
best choice most of the time.  For sheet views that contain more than 100
columns, an explicit "yes" or "no" improves performance.