toggleCheckbox method
Toggle a checkbox cell (boolean column) at (r, c). One undoable step.
Implementation
void toggleCheckbox(int r, int c) {
final col = columns[c];
if (col.type != EditableColumnType.checkbox || col.setValue == null) return;
final cur = truthy(col.value(_rows[r]));
final next = List<T>.from(_rows);
next[r] = col.setValue!(next[r], cur ? '0' : '1');
_sel = CellRef(r, c);
_apply(next);
}