Add help text to set as index image and fix its caching

Without this, the intent of the button is unclear. The image cached on
the gallery index page is also not updated when a new image is selected
as the index without the mtime provided by this change.

Author: aolley

edit/thumbnail/thumbnail.class.php

@@ -42,8 +42,14 @@ public function __construct($gallery, $cm, $image, $tab) {
      * @throws coding_exception
      */
     public function output() {
+        global $OUTPUT;
+
+        $url = new moodle_url('/mod/lightboxgallery/index.php', ['id' => $this->gallery->course]);
+        $helpbutton = $OUTPUT->help_icon('setasindex', 'lightboxgallery', true, $url);
         $result = '<input type="submit" class="btn btn-secondary" name="index" value="' .
-            get_string('setasindex', 'lightboxgallery') . '" /><br /><br />' .
+            get_string('setasindex', 'lightboxgallery') . '" />'.$helpbutton;
+
+        $result .= '<br /><br />' .
             get_string('selectthumbpos', 'lightboxgallery') . '<br /><br />';
 
         if ($this->lbgimage->width < $this->lbgimage->height) {

lang/en/lightboxgallery.php

@@ -138,6 +138,7 @@
 $string['selectrotation'] = 'Select rotation angle';
 $string['selectthumbpos'] = 'Thumbnail offset (from centre)';
 $string['setasindex'] = 'Set as index image';
+$string['setasindex_help'] = 'Selecting this sets this image as the primary image shown on the <a href="{$a}">list of lightbox galleries</a> in this course.';
 $string['showall'] = 'Show all';
 $string['sortby'] = 'Sort by';
 $string['sortby_help'] = 'How to sort the order the images are displayed in.';

locallib.php

@@ -248,8 +248,15 @@ function lightboxgallery_index_thumbnail($courseid, $gallery, $newimage = null)
         ];
         $index = $fs->create_file_from_pathname($fileinfo, $CFG->dirroot . '/mod/lightboxgallery/pix/index.png');
     }
-    $path = $CFG->wwwroot.'/pluginfile.php/'.$context->id.'/mod_lightboxgallery/gallery_index/'.
-                $index->get_itemid().$index->get_filepath().$index->get_filename();
+
+    $path = moodle_url::make_pluginfile_url($context->id,
+        'mod_lightboxgallery',
+        'gallery_index',
+        $index->get_itemid(),
+        $index->get_filepath(),
+        $index->get_filename(),
+    );
+    $path->param('mtime', $index->get_timemodified());
 
     return '<img src="' . $path . '" alt="" ' . (! empty($imageid) ? 'id="' . $imageid . '"' : '' )  . ' />';
 }