Add a Page::topParent() method to retrieve the topmost parent of a page

Adding to this branch as it's needed for the Admin change that handles
this branch change

system/src/Grav/Common/Page/Page.php

@@ -1716,6 +1716,31 @@ class Page
         return $pages->get($this->parent);
     }
 
+    /**
+     * Gets the top parent object for this page
+     *
+     * @return Page|null the top parent page object if it exists.
+     */
+    public function topParent()
+    {
+        $topParent = $this->parent();
+
+        if (!$topParent) {
+            return null;
+        }
+
+        while (true) {
+            $theParent = $topParent->parent();
+            if ($theParent != null && $theParent->parent() !== null) {
+                $topParent = $theParent;
+            } else {
+                break;
+            }
+        }
+
+        return $topParent;
+    }
+
     /**
      * Returns children of this page.
      *