add grid.draw for gglist to supporting ggsave

[xiangpin]

Now the ggsave can not save the gglist object.

> f %>% class
[1] "gglist" "list"  
> ggsave('./test.gglist.svg', f, device='svg')
Saving 7 x 7 in image
Error in UseMethod("grid.draw") : 
  "grid.draw"没有适用于"c('gglist', 'list')"目标对象的方法
> 

the pr fixed this issue.