Only allow to re-order tabs if we have more than 1

Otherwise it's confusing

Fixes: #27

src/private/widgets/TabBarWidget_p.h

@@ -52,6 +52,7 @@ public:
 protected:
     bool dragCanStart(QPoint pressPos, QPoint pos) const override;
     void mousePressEvent(QMouseEvent *) override;
+    void mouseMoveEvent(QMouseEvent *e) override;
 
 };
 }