LibWeb: Make the paint tree a proper standalone tree

Until now, paint trees have been piggybacking on the layout tree for
traversal, and paintables didn't actually have their own parent/child
pointers.

This patch changes that by making Paintable inherit from TreeNode, and
adding a new pass to LayoutState::commit() where we recursively build
the new paint tree.

Userland/Libraries/LibWeb/DOM/Document.cpp

@@ -1003,7 +1003,7 @@ void Document::update_layout()
                 Layout::AvailableSize::make_definite(viewport_rect.height())));
     }
 
-    layout_state.commit();
+    layout_state.commit(*m_layout_root);
 
     // Broadcast the current viewport rect to any new paintables, so they know whether they're visible or not.
     browsing_context()->inform_all_viewport_clients_about_the_current_viewport_rect();