Fixed MANTIS-4848 (Forced previews do not work correctly)

themes/default/javascript/checking.js

themes/default/templates/CATALOGUE_ADDING_SCREEN.tpl

themes/default/templates/CATALOGUE_EDITING_SCREEN.tpl

themes/default/templates/CONFIG_CATEGORY_SCREEN.tpl

themes/default/templates/FORM.tpl

themes/default/templates/FORM_GROUPED.tpl

themes/default/templates/FORM_SCREEN.tpl

themes/default/templates/MENU_EDITOR_SCREEN.tpl

themes/default/templates/POSTING_FORM.tpl

themes/default/templates/TEMPLATE_EDIT_SCREEN.tpl

themes/default/templates/THEME_EDIT_CSS_SCREEN.tpl

themes/default/templates/TRANSLATE_SCREEN.tpl

themes/default/templates/XML_CONFIG_SCREEN.tpl

themes/default/templates/ZONE_EDITOR_PANEL.tpl