MDL-75142 mod_data: Add description to presets

This commit adds support to description field for presets:
- For plugin presets, description will be stored in the module_help
string in lang files.
- For saved presets, a new element, <description>, has been added
to the preset.xml file, to include this information.

A new field has been added to the "Save as preset" modal, to let users
define description (which is empty by default, to support pre-existing
presets that won't have it).

Apart from that, the "Save as preset" workflow has been slightly improved,
following the Product UX/UX suggestions to hide the checkbox to overwrite
the preset by default and show it only if the user tries to save a preset
with an existing name.

mod/data/classes/manager.php

@@ -341,14 +341,7 @@ class manager {
                 continue;
             }
 
-            $preset = new stdClass();
-            $preset->isplugin = false;
-            $preset->path = $file->get_filepath();
-            $preset->name = trim($preset->path, '/');
-            $preset->shortname = $preset->name;
-            $preset->userid = $userid;
-            $preset->id = $file->get_id();
-            $preset->storedfile = $file;
+            $preset = preset::create_from_storedfile($this, $file);
             $presets[] = $preset;
         }
 
@@ -366,12 +359,7 @@ class manager {
         $dirs = core_component::get_plugin_list('datapreset');
         foreach ($dirs as $dir => $fulldir) {
             if (preset::is_directory_a_preset($fulldir)) {
-                $preset = new stdClass();
-                $preset->isplugin = true;
-                $preset->path = $fulldir;
-                $preset->userid = 0;
-                $preset->shortname = $dir;
-                $preset->name = preset::get_name_from_plugin($dir);
+                $preset = preset::create_from_plugin(null, $dir);
                 $presets[] = $preset;
             }
         }