feat: Don't fetch messages from unknown folders (#7190)

Actually this leads to fetching messages only from watched folders and Spam. Motivation:
- At least Gmail has virtual folders which aren't correctly detected as such, e.g. "Sent".
- At least Gmail has many virtual folders and scanning all of them takes significant time, 5-6 secs
  in median for me. This slows down receiving new messages and consumes battery.
- Delta Chat shouldn't fetch messages from folders potentially created by other apps for their own
  purposes. NB: All compatible Delta Chat forks should use the "DeltaChat" folder as mvbox.
- Fetching from folders that aren't watched, e.g. from "Sent", may lead to message ordering issues.

src/imap/scan_folders.rs

@@ -75,6 +75,7 @@ impl Imap {
             if !watched_folders.contains(&folder.name().to_string())
                 && folder_meaning != FolderMeaning::Drafts
                 && folder_meaning != FolderMeaning::Trash
+                && folder_meaning != FolderMeaning::Unknown
             {
                 self.fetch_move_delete(context, session, folder.name(), folder_meaning)
                     .await