Check if I need new UUIDs for scraps when cloning a page

[adamwulf]

Related to #501

I do change the scrap UUIDs when sending a page over cloud kit, but I don't change the UUIDs when I clone scraps to/from other pages or the sidebar.

That means that I /do/ need to change the scrap UUID when I clone a page. otherwise I could clone a page, and then move that 2nd page's scraps back to the first page and have a scrap UUID conflict.

• abstract out the UUID update code from MMExportablePaperView.m so that I can change a scraps UUID from either sending over cloudkit or when cloning a page without duplicating code.

[adamwulf]

I take all of that back. It actually clones the scrap to the new page when adding from the sidebar, so it doesn't matter if two pages have scraps with the exact same UUID after all.