Follow-on to 1e4d75c120 for #1975.

Create a siblings() function which pulls together the siblings_callback
function and makes it a more palatable API.

modules/gallery/libraries/Theme_View.php

@@ -86,6 +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 tag() {
     return $this->tag;
   }