Copy and paste works across editor pages

The editor's copy lived only in the page's memory. Leaving edit mode or opening another page wiped it, and the most common request, copying a section from here and pasting it there, was impossible.

The copy is now kept in the browser and works across every page of the same site, in the live editor and in the panel editor, and between open tabs.

Before being stored, the block is cleaned of what only makes sense on the source page. Widget markers are kept, and identifiers are rebuilt on paste, so the copy does not collide with the original on the same page.