File widgets

cnc-file-tree and cnc-file-browser never touch storage. They draw what they are handed, update their own view at once, and dispatch an event saying what the reader asked for. The page does the write: here, one handler each for a rename, a move and an upload, in filing.mjs.

Nothing yet: rename a row, drag one, or upload a file.

Drag a row into read-only to watch rollback() put it back: the widget had already moved it, and this page refused the write.