Don't auto populate QgsLayoutItemRegistry

Initially create an empty registry, allow it to be populated
at a later stage.

python/core/layout/qgslayoutitemregistry.sip

@@ -110,14 +110,23 @@ class QgsLayoutItemRegistry : QObject
 
     QgsLayoutItemRegistry( QObject *parent = 0 );
 %Docstring
- Creates a registry and populates it with standard item types.
+ Creates a new empty item registry.
 
  QgsLayoutItemRegistry is not usually directly created, but rather accessed through
  QgsApplication.layoutItemRegistry().
+
+.. seealso:: populate()
 %End
 
     ~QgsLayoutItemRegistry();
 
+    bool populate();
+%Docstring
+ Populates the registry with standard item types. If called on a non-empty registry
+ then this will have no effect and will return false.
+ :rtype: bool
+%End
+
 
     QgsLayoutItemAbstractMetadata *itemMetadata( int type ) const;
 %Docstring

src/core/layout/qgslayoutitemregistry.cpp

@@ -20,6 +20,17 @@
 QgsLayoutItemRegistry::QgsLayoutItemRegistry( QObject *parent )
   : QObject( parent )
 {
+}
+
+QgsLayoutItemRegistry::~QgsLayoutItemRegistry()
+{
+  qDeleteAll( mMetadata );
+}
+
+bool QgsLayoutItemRegistry::populate()
+{
+  if ( !mMetadata.isEmpty() )
+    return false;
 
   // add temporary item to register
   auto createTemporaryItem = []( QgsLayout * layout, const QVariantMap & )->QgsLayoutItem*
@@ -28,11 +39,7 @@ QgsLayoutItemRegistry::QgsLayoutItemRegistry( QObject *parent )
   };
 
   addLayoutItemType( new QgsLayoutItemMetadata( 101, QStringLiteral( "temp type" ), QgsApplication::getThemeIcon( QStringLiteral( "/mActionAddLabel.svg" ) ), createTemporaryItem ) );
-}
-
-QgsLayoutItemRegistry::~QgsLayoutItemRegistry()
-{
-  qDeleteAll( mMetadata );
+  return true;
 }
 
 QgsLayoutItemAbstractMetadata *QgsLayoutItemRegistry::itemMetadata( int type ) const

src/core/layout/qgslayoutitemregistry.h

@@ -232,15 +232,23 @@ class CORE_EXPORT QgsLayoutItemRegistry : public QObject
     };
 
     /**
-     * Creates a registry and populates it with standard item types.
+     * Creates a new empty item registry.
      *
      * QgsLayoutItemRegistry is not usually directly created, but rather accessed through
      * QgsApplication::layoutItemRegistry().
+     *
+     * \see populate()
     */
     QgsLayoutItemRegistry( QObject *parent = nullptr );
 
     ~QgsLayoutItemRegistry();
 
+    /**
+     * Populates the registry with standard item types. If called on a non-empty registry
+     * then this will have no effect and will return false.
+     */
+    bool populate();
+
     //! QgsLayoutItemRegistry cannot be copied.
     QgsLayoutItemRegistry( const QgsLayoutItemRegistry &rh ) = delete;
     //! QgsLayoutItemRegistryQgsLayoutItemRegistry cannot be copied.

src/core/qgsapplication.cpp

@@ -1596,6 +1596,7 @@ QgsApplication::ApplicationMembers::ApplicationMembers()
   mProcessingRegistry = new QgsProcessingRegistry();
   mPageSizeRegistry = new QgsPageSizeRegistry();
   mLayoutItemRegistry = new QgsLayoutItemRegistry();
+  mLayoutItemRegistry->populate();
   mProcessingRegistry->addProvider( new QgsNativeAlgorithms( mProcessingRegistry ) );
   mAnnotationRegistry = new QgsAnnotationRegistry();
 }

tests/src/core/testqgslayoutitem.cpp

@@ -159,6 +159,13 @@ void TestQgsLayoutItem::registry()
   QCOMPARE( props.size(), 1 );
   registry.resolvePaths( 2, props, QgsPathResolver(), true );
   QVERIFY( props.isEmpty() );
+
+  //test populate
+  QgsLayoutItemRegistry reg2;
+  QVERIFY( reg2.itemTypes().isEmpty() );
+  QVERIFY( reg2.populate() );
+  QVERIFY( !reg2.itemTypes().isEmpty() );
+  QVERIFY( !reg2.populate() );
 }
 
 bool TestQgsLayoutItem::renderCheck( QString testName, QImage &image, int mismatchCount )