Fix #11169 (toggling symbol visibility does not trigger refresh)

This could have happened when the same project was reloaded, then map canvas
failed to clear itself and some connections to repaintRequested were broken

src/app/qgisapp.cpp

@@ -2327,7 +2327,7 @@ void QgisApp::initLayerTreeView()
   // visibility groups tool button
   QToolButton* btnVisibilityGroups = new QToolButton;
   btnVisibilityGroups->setAutoRaise( true );
-  btnVisibilityGroups->setToolTip( tr( "Manage Layer Visibility") );
+  btnVisibilityGroups->setToolTip( tr( "Manage Layer Visibility" ) );
   btnVisibilityGroups->setIcon( QgsApplication::getThemeIcon( "/mActionShowAllLayers.png" ) );
   btnVisibilityGroups->setPopupMode( QToolButton::InstantPopup );
   btnVisibilityGroups->setMenu( QgsVisibilityGroups::instance()->menu() );
@@ -7848,6 +7848,9 @@ void QgisApp::closeProject()
   removeAnnotationItems();
   // clear out any stuff from project
   mMapCanvas->freeze( true );
+  QList<QgsMapCanvasLayer> emptyList;
+  mMapCanvas->setLayerSet( emptyList );
+  mMapCanvas->clearCache();
   removeAllLayers();
 }