LibWeb: Make CSS layout lazier

Instead of doing layout synchronously whenever something changes,
we now use a basic event loop timer to defer and coalesce relayouts.

If you did something that requires a relayout of the page, make sure
to call Document::set_needs_layout() and it will get coalesced with all
the other layout updates.

There's lots of room for improvement here, but this already makes many
web pages significantly snappier. :^)

Also, note that this exposes a number of layout bugs where we have been
relying on multiple relayouts to calculate the correct dimensions for
things. Now that we only do a single layout in many cases, these kind of
problems are much more noticeable. That should also make them easier to
figure out and fix. :^)

Userland/DevTools/HackStudio/Debugger/EvaluateExpressionDialog.cpp

@@ -144,8 +144,6 @@ void EvaluateExpressionDialog::set_output(const StringView& html)
     paragraph->set_inner_html(html);
 
     m_output_container->append_child(paragraph);
-    m_output_container->document().invalidate_layout();
-    m_output_container->document().update_layout();
 }
 
 }