As shown in FIG. 2, FIG. 4A and FIG. 4B, in the embodiment, the second frame bar 200 is formed with a first fixing slot 240 and a second fixing slot 250. The first fixing slot 240 and the second fixing slot 250 are respectively formed on two opposite inner walls of the second frame bar 200 respectively facing with each other, that is, the second inlet 230 is formed between the first fixing slot 240 and the second fixing slot 250. The first fixing slot 240 and the second fixing slot 250 are vertically staggered from each other. The first fixing slot 240 and the first sliding groove 140 are commonly located at the first height H1, and the second fixing slot 250 and the second sliding groove 150 are commonly located at the second height H2.