fix: Fix transfer history clearing in-progress files too early

We recently upgraded SyncTrayzor's understanding of folder states, e.g.
by adding the various -waiting states that SyncTrayzor previously did
not understand.

However, syncthing can flip-flop in between these states, for example by
briefly going to syncing, then preparing (sync-waiting) and then back to
syncing.

Previously SyncTrayzor assumed that any non-syncing state meant that
syncthing had completed any file transfers. This is no longer true with
the various waiting states. Rather, only consider a folder as completed
if it actually is idle (errors are handled seperately).

Author: GermanCoding

src/SyncTrayzor/Syncthing/TransferHistory/SyncthingTransferHistory.cs

@@ -229,7 +229,7 @@ private void SyncStateChanged(object sender, FolderSyncStateChangedEventArgs e)
         {
             var folderId = e.FolderId;
 
-            if (e.PrevSyncState == FolderSyncState.Syncing)
+            if (e.SyncState == FolderSyncState.Idle)
             {
                 List<FileTransfer> transferredList = null;
                 List<FileTransfer> completedFileTransfers; // Those that Syncthing didn't tell us had completed