Use native sketch visibility

Gets rid of the "&" business. Exports all hidden layers (use '-' to explicitly ignore). Uses native layer visibility to decide whether View in Framer will be hidden.

Also fixes framerps.js so layers without background art are also correctly set as visible.

I have to make the changes in the layer itself, not the copy. Because if a layer group is invisible, its children will remain invisible unless I make the parent temporarily visible.