Drop fields in correct position in drag and drop designer

Instead of always dropping fields at the end of the container,
insert them at the dropped location. Otherwise the drop indicator
line is misleading.

src/app/qgsattributesformproperties.cpp

@@ -786,7 +786,7 @@ DnDTree::DnDTree( QgsVectorLayer *layer, QWidget *parent )
   connect( this, &QTreeWidget::itemDoubleClicked, this, &DnDTree::onItemDoubleClicked );
 }
 
-QTreeWidgetItem *DnDTree::addItem( QTreeWidgetItem *parent, QgsAttributesFormProperties::DnDTreeItemData data )
+QTreeWidgetItem *DnDTree::addItem( QTreeWidgetItem *parent, QgsAttributesFormProperties::DnDTreeItemData data, int index )
 {
   QTreeWidgetItem *newItem = new QTreeWidgetItem( QStringList() << data.name() );
   newItem->setFlags( Qt::ItemIsEnabled | Qt::ItemIsSelectable | Qt::ItemIsDragEnabled );
@@ -811,7 +811,10 @@ QTreeWidgetItem *DnDTree::addItem( QTreeWidgetItem *parent, QgsAttributesFormPro
     }
   }
   newItem->setData( 0, QgsAttributesFormProperties::DnDTreeRole, data );
-  parent->addChild( newItem );
+  if ( index < 0 )
+    parent->addChild( newItem );
+  else
+    parent->insertChild( index, newItem );
 
   return newItem;
 }
@@ -850,7 +853,6 @@ void DnDTree::dragMoveEvent( QDragMoveEvent *event )
 
 bool DnDTree::dropMimeData( QTreeWidgetItem *parent, int index, const QMimeData *data, Qt::DropAction action )
 {
-  Q_UNUSED( index )
   bool bDropSuccessful = false;
 
   if ( action == Qt::IgnoreAction )
@@ -869,12 +871,12 @@ bool DnDTree::dropMimeData( QTreeWidgetItem *parent, int index, const QMimeData
 
       if ( parent )
       {
-        addItem( parent, itemElement );
+        addItem( parent, itemElement, index );
         bDropSuccessful = true;
       }
       else
       {
-        addItem( invisibleRootItem(), itemElement );
+        addItem( invisibleRootItem(), itemElement, index );
         bDropSuccessful = true;
       }
     }

src/app/qgsattributesformproperties.h

@@ -238,7 +238,12 @@ class DnDTree : public QTreeWidget
 
   public:
     explicit DnDTree( QgsVectorLayer *layer, QWidget *parent = nullptr );
-    QTreeWidgetItem *addItem( QTreeWidgetItem *parent, QgsAttributesFormProperties::DnDTreeItemData data );
+
+    /**
+     * Adds a new item to a \a parent. If \a index is -1, the item is added to the end of the parent's existing children.
+     * Otherwise it is inserted at the specified \a index.
+     */
+    QTreeWidgetItem *addItem( QTreeWidgetItem *parent, QgsAttributesFormProperties::DnDTreeItemData data, int index = -1 );
     QTreeWidgetItem *addContainer( QTreeWidgetItem *parent, const QString &title, int columnCount );
 
     enum Type