Don't duplicate layer tree node when alt is held

Looks like it duplicates the layer but it really just copies the node
meaning QGIS can get confused

Duplicate on Alt will come at a later date

src/gui/layertree/qgslayertreeview.cpp

@@ -408,3 +408,12 @@ void QgsLayerTreeView::keyPressEvent( QKeyEvent *event )
   QTreeView::keyPressEvent( event );
   layerTreeModel()->setFlags( oldFlags );
 }
+
+void QgsLayerTreeView::dropEvent( QDropEvent *event )
+{
+  if ( event->keyboardModifiers() & Qt::AltModifier )
+  {
+    event->accept();
+  }
+  QTreeView::dropEvent( event );
+}

src/gui/layertree/qgslayertreeview.h

@@ -127,6 +127,9 @@ class GUI_EXPORT QgsLayerTreeView : public QTreeView
     void mouseReleaseEvent( QMouseEvent *event ) override;
     void keyPressEvent( QKeyEvent *event ) override;
 
+    void dropEvent( QDropEvent *event ) override;
+
+
   protected slots:
 
     void modelRowsInserted( const QModelIndex &index, int start, int end );