MDL-31989 search: Search API and search engine API

Introducing both APIs in moodle along with:
- search_box widget to add a tiny search box
- admin settings with setup steps helper
- cache for search results
- template for a search result
- php unit stuff

Many thanks to Tomasz Muras, Prateek Sachan and Daniel Neis for their contributions, for starting this development
and for pushing for it to be completed. Also thanks to other contributors: Jonathan Harker and eugeneventer.

admin/settings/plugins.php

@@ -457,6 +457,47 @@ foreach ($pages as $page) {
     $ADMIN->add('reportplugins', $page);
 }
 
+// Global Search engine plugins.
+$ADMIN->add('modules', new admin_category('searchplugins', new lang_string('search', 'admin')));
+$temp = new admin_settingpage('manageglobalsearch', new lang_string('globalsearchmanage', 'admin'));
+
+$pages = array();
+$engines = array();
+foreach (core_component::get_plugin_list('search') as $engine => $plugindir) {
+    $engines[$engine] = new lang_string('pluginname', 'search_' . $engine);
+    $settingspath = "$plugindir/settings.php";
+    if (file_exists($settingspath)) {
+        $settings = new admin_settingpage('search' . $engine,
+                new lang_string('pluginname', 'search_' . $engine), 'moodle/site:config');
+        include($settingspath);
+        if ($settings) {
+            $pages[] = $settings;
+        }
+    }
+}
+
+// Setup status.
+$temp->add(new admin_setting_searchsetupinfo());
+
+// Search engine selection.
+$temp->add(new admin_setting_heading('searchengineheading', new lang_string('searchengine', 'admin'), ''));
+$temp->add(new admin_setting_configselect('searchengine',
+                            new lang_string('selectsearchengine', 'admin'), '', 'solr', $engines));
+
+// Enable search areas.
+$temp->add(new admin_setting_heading('searchareasheading', new lang_string('availablesearchareas', 'admin'), ''));
+$searchareas = \core_search\manager::get_search_areas_list();
+foreach ($searchareas as $areaid => $searcharea) {
+    list($componentname, $varname) = $searcharea->get_config_var_name();
+    $temp->add(new admin_setting_configcheckbox($componentname . '/enable' . $varname, $searcharea->get_visible_name(true),
+        '', 1, 1, 0));
+}
+$ADMIN->add('searchplugins', $temp);
+
+foreach ($pages as $page) {
+    $ADMIN->add('searchplugins', $page);
+}
+
 /// Add all admin tools
 if ($hassiteconfig) {
     $ADMIN->add('modules', new admin_category('tools', new lang_string('tools', 'admin')));