Support re-ordering tabs with mouse

Just set KDDockWidgets::Config::Flag_AllowReorderTabs before creating
the dock widgets.

Fixes #20

src/private/DragController.cpp

@@ -155,17 +155,17 @@ bool StatePreDrag::handleMouseMove(QPoint globalPos)
 {
     if (q->m_draggable->dragCanStart(q->m_pressPos, globalPos)) {
         Q_EMIT q->manhattanLengthMove();
+        return true;
     }
-    return true;
+    return false;
 }
 
 bool StatePreDrag::handleMouseButtonRelease(QPoint)
 {
     Q_EMIT q->dragCanceled();
-    return true;
+    return false;
 }
 
-
 StateDragging::StateDragging(DragController *parent)
     : StateBase(parent)
 {