Hide 'duplicate features' actions by default

Since there's a number of serious outstanding issues with these
tools, hide them by default and show them only if a QgsSettings
flag is set ("tools\showDuplicateFeatureActions").

This allows the tools to be reworked for 3.2, while still
making them accessible for users who require their functionality.

resources/qgis_global_settings.ini

@@ -33,3 +33,8 @@ connections-xyz\OpenStreetMap\zmin=0
 # Default help location to include
 # for now this is online version of the User Guide for latest (LTR) release
 helpSearchPath=https://docs.qgis.org/$qgis_short_version/$qgis_locale/docs/user_manual/
+
+[app]
+
+# If true, the experimental "duplicate feature" actions will be shown in the QGIS UI
+tools\showDuplicateFeatureActions=false

src/app/qgisapp.cpp

@@ -7488,6 +7488,9 @@ void QgisApp::setupLayoutManagerConnections()
 }
 
 void QgisApp::setupDuplicateFeaturesAction()
+{
+  QgsSettings settings;
+  if ( settings.value( QStringLiteral( "tools/showDuplicateFeatureActions" ), false, QgsSettings::App ).toBool() )
   {
     mDuplicateFeatureAction.reset( new QgsMapLayerAction( tr( "Duplicate feature" ),
                                    nullptr, QgsMapLayerAction::SingleFeature,
@@ -7511,6 +7514,7 @@ void QgisApp::setupDuplicateFeaturesAction()
     }
            );
   }
+}
 
 void QgisApp::setupAtlasMapLayerAction( QgsPrintLayout *layout, bool enableAction )
 {