MDL-59165 output: Add standard footer html callback

lib/outputrenderers.php

@@ -741,16 +741,26 @@ class core_renderer extends renderer_base {
     public function standard_footer_html() {
         global $CFG, $SCRIPT;
 
+        $output = '';
         if (during_initial_install()) {
             // Debugging info can not work before install is finished,
             // in any case we do not want any links during installation!
-            return '';
+            return $output;
+        }
+
+        // Give plugins an opportunity to add any footer elements.
+        // The callback must always return a string containing valid html footer content.
+        $pluginswithfunction = get_plugins_with_function('standard_footer_html', 'lib.php');
+        foreach ($pluginswithfunction as $plugins) {
+            foreach ($plugins as $function) {
+                $output .= $function();
+            }
         }
 
         // This function is normally called from a layout.php file in {@link core_renderer::header()}
         // but some of the content won't be known until later, so we return a placeholder
         // for now. This will be replaced with the real content in {@link core_renderer::footer()}.
-        $output = $this->unique_performance_info_token;
+        $output .= $this->unique_performance_info_token;
         if ($this->page->devicetypeinuse == 'legacy') {
             // The legacy theme is in use print the notification
             $output .= html_writer::tag('div', get_string('legacythemeinuse'), array('class'=>'legacythemeinuse'));