The first fastening groove 121 may be recessed from a surface toward the other surface and may be formed to correspond to the shape of the first fastening protrusion (171, see FIG. 2A) of the connecting bracket (150, see FIG. 2A). The first fastening groove 121 may be recessed from a surface toward the other surface without passing through the fixing bracket 110. The second fastening groove 123 may extend in the width direction of the fixing bracket 110 at the end of the other surface of the first fastening groove 121 and may be formed to correspond to the shape of the second fastening protrusion (173, see FIG. 2A) of the connecting bracket (150, see FIG. 2A). The second fastening groove 123 may also extend in the width direction of the fixing bracket 110 without passing through the fixing bracket 110 in the width direction. That is, the fastening groove may be formed in a T-shape when seen from the top in the state shown in FIG. 2.

The fastening groove may not pass through the fixing bracket 110 in the up-down direction. Accordingly, it is possible to prevent the connecting bracket (150, see FIG. 2A) from being separated through the fixing bracket 110 due to the gravity when the connecting bracket (150, see FIG. 2A) is inserted in the fastening groove 120 of the fixing bracket 110.