Switch FolderPath and ImapDB.Folder's placeholder hierarchy delimiter.

Since FolderPath's gets displayed to the end user, use something a bit
less crap than a '?'.

src/engine/api/geary-folder-path.vala

@@ -310,7 +310,7 @@ public class Geary.FolderPath : BaseObject, Gee.Hashable<Geary.FolderPath>,
      * Use only for debugging and logging.
      */
     public string to_string() {
-        return get_fullpath("?");
+        return get_fullpath(">");
     }
 }
 

src/engine/imap-db/imap-db-folder.vala

@@ -190,7 +190,7 @@ private class Geary.ImapDB.Folder : BaseObject, Geary.ReferenceSemantics {
                 : null;
             
             // Note that recent is not stored
-            status_data = new Imap.StatusData(new Imap.MailboxSpecifier.from_folder_path(path, ">"),
+            status_data = new Imap.StatusData(new Imap.MailboxSpecifier.from_folder_path(path, "?"),
                 messages, 0, uid_next, uid_validity, result.int_for("unread_count"));
             
             return Db.TransactionOutcome.DONE;