is a plan view of the structure of Fig. 8; Fig. 10 is a sectional view on an enlarged scale taken along line X-X of Fig. 8 in the direction of the arrows; Fig. 11 is an elevational view of a trimming means according to the present invention; Fig. 12 is a plan view of the structure of Fig. 11; Fig. 13 is a sectional view on an enlarged scale taken along line XIIIXIII of Fig. 11 in the direction of