Add 'Moving' state for torrents being relocated/moved

This is another indication to the user that something is happening
behind the scenes.
Uses the same icon/color as "Checking" status.
Torrents in the `Moving` state are considered as "Active". This should
prevent accidental program exit from the user and inhibit system sleep.

src/webui/api/serialize/serialize_torrent.cpp

@@ -76,6 +76,8 @@ namespace
 #endif
         case BitTorrent::TorrentState::CheckingResumeData:
             return QLatin1String("checkingResumeData");
+        case BitTorrent::TorrentState::Moving:
+            return QLatin1String("moving");
         default:
             return QLatin1String("unknown");
         }