LibWeb: Enable partial layout tree update in a bunch of cases

These common cases now cause us to invalidate the layout tree starting
at the relevant parent node instead of invalidating the entire tree.

- DOM node insertion
- DOM node removal
- innerHTML setter
- textContent setter

This makes a lot of dynamic content much faster. For example, demos
on shadertoy.com go from ~18 fps to ~28 fps on my machine.

Libraries/LibWeb/DOM/Element.cpp

@@ -808,7 +808,7 @@ WebIDL::ExceptionOr<void> Element::set_inner_html(StringView value)
 
         if (context->is_connected()) {
             // NOTE: Since the DOM has changed, we have to rebuild the layout tree.
-            context->document().invalidate_layout_tree();
+            context->set_needs_layout_tree_update(true);
         }
     }