LibWeb: More thoroughly detach layout/paint trees from each other

Before committing a new layout (and thus building a new paint tree)
we now go through both the old paint tree and the layout tree and detach
them from each other.

This is a little extra work, but it ensures that there are no lingering
references across the trees, which we were apparently accumulating in
some cases on Discord, causing GC leaks.

Libraries/LibWeb/Painting/Paintable.cpp

@@ -36,6 +36,12 @@ Paintable::~Paintable()
 {
 }
 
+void Paintable::detach_from_layout_node()
+{
+    if (m_list_node.is_in_list())
+        m_list_node.remove();
+}
+
 void Paintable::visit_edges(Cell::Visitor& visitor)
 {
     Base::visit_edges(visitor);