[layouts] Add search filter box to 'add layer to legend' dialog

Helps with finding specific layers in large projects

src/app/layout/qgslayoutlegendlayersdialog.cpp

@@ -27,12 +27,17 @@ QgsLayoutLegendLayersDialog::QgsLayoutLegendLayersDialog( QWidget *parent )
   QgsSettings settings;
   restoreGeometry( settings.value( QStringLiteral( "Windows/LayoutLegendLayers/geometry" ) ).toByteArray() );
 
+  mFilterLineEdit->setShowClearButton( true );
+  mFilterLineEdit->setShowSearchIcon( true );
+
   mModel = new QgsMapLayerProxyModel( listMapLayers );
   listMapLayers->setModel( mModel );
   QModelIndex firstLayer = mModel->index( 0, 0 );
   listMapLayers->selectionModel()->select( firstLayer, QItemSelectionModel::Select );
 
   connect( listMapLayers, &QListView::doubleClicked, this, &QgsLayoutLegendLayersDialog::accept );
+
+  connect( mFilterLineEdit, &QLineEdit::textChanged, mModel, &QgsMapLayerProxyModel::setFilterString );
 }
 
 QgsLayoutLegendLayersDialog::~QgsLayoutLegendLayersDialog()

src/ui/layout/qgslayoutlegendlayersdialogbase.ui

@@ -14,7 +14,10 @@
    <string>Add Layer to Legend</string>
   </property>
   <layout class="QGridLayout" name="gridLayout">
-   <item row="1" column="0">
+   <property name="spacing">
+    <number>6</number>
+   </property>
+   <item row="2" column="0">
     <widget class="QDialogButtonBox" name="buttonBox">
      <property name="orientation">
       <enum>Qt::Horizontal</enum>
@@ -24,15 +27,33 @@
      </property>
     </widget>
    </item>
-   <item row="0" column="0">
+   <item row="1" column="0">
     <widget class="QListView" name="listMapLayers">
      <property name="selectionMode">
       <enum>QAbstractItemView::ExtendedSelection</enum>
      </property>
     </widget>
    </item>
+   <item row="0" column="0">
+    <widget class="QgsFilterLineEdit" name="mFilterLineEdit">
+     <property name="placeholderText">
+      <string>Search</string>
+     </property>
+    </widget>
+   </item>
   </layout>
  </widget>
+ <customwidgets>
+  <customwidget>
+   <class>QgsFilterLineEdit</class>
+   <extends>QLineEdit</extends>
+   <header>qgsfilterlineedit.h</header>
+  </customwidget>
+ </customwidgets>
+ <tabstops>
+  <tabstop>mFilterLineEdit</tabstop>
+  <tabstop>listMapLayers</tabstop>
+ </tabstops>
  <resources/>
  <connections>
   <connection>