Adds a new row of cells below the last row.
- macOS 10.0+
If the number of rows or columns in the receiver has been changed with
renew, then new cells are created only if they are needed. This fact allows you to grow and shrink an NSMatrix without repeatedly creating and freeing the cells.
This method redraws the receiver. Your code may need to send
size after sending this method to resize the receiver to fit the newly added cells.