QmlDesigner: Optimize adding QtQuick3D and asset imports

Update subcomponentmanager only for the needed imports when:
- Adding the QtQuick3D import via the link in the 3D editor.
- Adding 3D asset using the assets importer.

Also few renamings to make things clearer.

Change-Id: I4eb55f64307498f3dbb400e947667dfe9e42ff8e
Reviewed-by: Thomas Hartmann <thomas.hartmann@qt.io>

src/plugins/qmldesigner/components/edit3d/edit3dview.cpp

@@ -319,7 +319,8 @@ void Edit3DView::addQuick3DImport()
                 model()->changeImports({import}, {});
 
                 // Subcomponent manager update needed to make item library entries appear
-                QmlDesignerPlugin::instance()->currentDesignDocument()->updateSubcomponentManager();
+                QmlDesignerPlugin::instance()->currentDesignDocument()
+                                             ->updateSubcomponentManagerImport(import);
                 return;
             }
         }