Add cancellation reason tracking with user/system codes and UI

internal/syncengine/queue_test.go

@@ -33,10 +33,15 @@ func TestQueue(t *testing.T) {
 		t.Errorf("Enqueue(1) again = %v, want ErrAlreadyRunning", err)
 	}
 
-	q.Cancel(1)
+	q.Cancel(1, true)
 	if !cancelCalled {
 		t.Error("Cancel should have called the cancel func")
 	}
+	if !q.IsCancelledByUser(1) {
+		t.Error("IsCancelledByUser should return true after Cancel(1, true)")
+	}
+
+	q.Dequeue(1)
 
 	q.Dequeue(1)
 	if q.IsRunning(1) {