Extend siblings callbacks to take a $limit and an $offset for navigating

large sibling sets.  Useful for the thumbnav module since we don't want to
iterate a thousand siblings to find the one we care about.  Fixes #1999.

modules/gallery/controllers/albums.php

@@ -98,9 +98,9 @@ class Albums_Controller extends Items_Controller {
                  "breadcrumbs" => Breadcrumb::array_from_item_parents($item));
   }
 
-  static function get_siblings($item) {
+  static function get_siblings($item, $limit=null, $offset=null) {
     // @todo consider creating Item_Model::siblings() if we use this more broadly.
-    return $item->parent()->viewable()->children();
+    return $item->parent()->viewable()->children($limit, $offset);
   }
 
   public function create($parent_id) {

modules/gallery/libraries/Theme_View.php

@@ -86,8 +86,10 @@ class Theme_View_Core extends Gallery_View {
     return $this->item;
   }
 
-  public function siblings() {
-    return call_user_func_array($this->siblings_callback[0], $this->siblings_callback[1]);
+  public function siblings($limit=null, $offset=null) {
+    return call_user_func_array(
+      $this->siblings_callback[0],
+      array_merge($this->siblings_callback[1], array($offset, $limit)));
   }
 
   public function tag() {

modules/search/controllers/search.php

@@ -110,8 +110,8 @@ class Search_Controller extends Controller {
                    Breadcrumb::instance($item->title, $item->url())->set_last()));
   }
 
-  static function get_siblings($q, $album) {
-    $result = search::search_within_album(search::add_query_terms($q), $album, 1000, 1);
+  static function get_siblings($q, $album, $limit=1000, $offset=1) {
+    $result = search::search_within_album(search::add_query_terms($q), $album, $limit, $offset);
     return $result[1];
   }
 }