Add BEFORE_EXPORT event and fix bug on modal
	modifié:         core/lib/Thelia/Controller/Admin/ImportExportController.php
	nouveau fichier: core/lib/Thelia/Core/Event/ImportExport/Export.php
	modifié:         core/lib/Thelia/Core/Event/TheliaEvents.php
	modifié:         core/lib/Thelia/Core/FileFormat/Formatting/Formatter/XMLFormatter.php
	modifié:         templates/backOffice/default/ajax/export-modal.html
	modifié:         templates/backOffice/default/export-page.html