fix bottom_line

src/core/js/ext_bottom_line.js

@@ -395,14 +395,13 @@ var ext_bottom_line = function($, logger, is_in_view_port, load_config, getEntit
      * @return {HTMLElement} the newly created container.
      */
     var add_preview_container = function(entity) {
-        const button_show = $('<button class="btn btn-sm card-footer"><span class="glyphicon glyphicon-menu-down"/> Show Preview</button>')
+        const button_show = $('<button class="btn btn-sm card-footer"><i class="bi bi-chevron-down"></i> Show Preview</button>')
             .css({
                 width: "100%",
                 padding: "0",
             })
             .addClass(_css_class_preview_container_button)
-            .hide();
-        const button_hide = $('<button class="btn btn-sm card-footer"><span class="glyphicon glyphicon-menu-up"/> Hide Preview</button>')
+        const button_hide = $('<button class="btn btn-sm card-footer"><i class="bi bi-chevron-up"></i> Hide Preview</button>')
             .css({
                 width: "100%",
                 padding: "0",