The standard UI way of doing it would probably be to add an eye icon to each item/row. In our case it wouldn't affect anything in the actual layout used in the app, just whether or not that item is displayed in the visual designer.
I took a picture from 
this post:
And opened the 
image editor GIMP and remade the rows in it. (Ignore the "icon" to the left of the name, that's obviously just for show in GIMP.) See the eye icon to the left, where one can toggle visibility of layers/rows on and off.
That said, it's usually frowned upon to have very large and complex layouts. Instead we are encouraged to break them up into smaller layouts that we merge in the app.