[layouts] Sort layers by name in 'add layer' to legend dialog

Because it's hard to find specific layers when the layers are
listed in project order

src/app/layout/qgslayoutlegendlayersdialog.cpp

@@ -17,6 +17,7 @@
 #include <QStandardItem>
 #include "qgsmaplayer.h"
 #include "qgsmaplayermodel.h"
+#include "qgsmaplayerproxymodel.h"
 #include "qgssettings.h"
 
 QgsLayoutLegendLayersDialog::QgsLayoutLegendLayersDialog( QWidget *parent )
@@ -26,7 +27,7 @@ QgsLayoutLegendLayersDialog::QgsLayoutLegendLayersDialog( QWidget *parent )
   QgsSettings settings;
   restoreGeometry( settings.value( QStringLiteral( "Windows/LayoutLegendLayers/geometry" ) ).toByteArray() );
 
-  mModel = new QgsMapLayerModel( listMapLayers );
+  mModel = new QgsMapLayerProxyModel( listMapLayers );
   listMapLayers->setModel( mModel );
   QModelIndex firstLayer = mModel->index( 0, 0 );
   listMapLayers->selectionModel()->select( firstLayer, QItemSelectionModel::Select );
@@ -47,7 +48,13 @@ QList< QgsMapLayer *> QgsLayoutLegendLayersDialog::selectedLayers() const
   const QModelIndexList selection = listMapLayers->selectionModel()->selectedIndexes();
   for ( const QModelIndex &index : selection )
   {
-    QgsMapLayer *layer = mModel->layerFromIndex( index );
+    const QModelIndex sourceIndex = mModel->mapToSource( index );
+    if ( !sourceIndex.isValid() )
+    {
+      continue;
+    }
+
+    QgsMapLayer *layer = mModel->sourceLayerModel()->layerFromIndex( sourceIndex );
     if ( layer )
       layers << layer;
   }

src/app/layout/qgslayoutlegendlayersdialog.h

@@ -18,7 +18,7 @@
 #include "ui_qgslayoutlegendlayersdialogbase.h"
 
 class QgsMapLayer;
-class QgsMapLayerModel;
+class QgsMapLayerProxyModel;
 
 /**
  * \ingroup app
@@ -37,7 +37,7 @@ class QgsLayoutLegendLayersDialog: public QDialog, private Ui::QgsLayoutLegendLa
   private:
     QgsLayoutLegendLayersDialog() = delete;
 
-    QgsMapLayerModel *mModel = nullptr;
+    QgsMapLayerProxyModel *mModel = nullptr;
 };
 
 #endif //QGSLAYOUTLEGENDLAYERSDIALOG_H