feat(core,fuse,cli,gui): discard a queued change and undo it here

A change Drive will not take could be exported or retried, never let go
of. The Sync page's Discard button (with a confirmation) and
`pdfs sync discard <id>` drop it: a node made here goes, as if deleted
before it was sent; a node Drive has goes back to what Drive has, under
the same inode, with every change queued on it, while what is queued
below it stays. A change on the wire or an open file is refused.

docs/USER_GUIDE.md

@@ -167,6 +167,8 @@ The version you do not keep goes to the Trash, so a wrong choice can be undone.
 - **Limit bandwidth** under **Preferences → General → Network**, or with
   `pdfs sync limit --up 2M --down 10M`. `0` removes a limit.
 - **Retry** a failed upload from **Sync → Overview**, or with `pdfs sync retry`.
+- **Discard** a change Proton Drive refused from the same list, or with `pdfs sync discard`: it is
+  undone here and never sent. Export a file first to keep your version.
 
 ## Photos