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/Libraries/LibWeb/DOM/CharacterData.cpp

@@ -24,8 +24,7 @@ void CharacterData::set_data(String data)
     if (m_data == data)
         return;
     m_data = move(data);
-    // FIXME: This is definitely too aggressive.
-    document().schedule_forced_layout();
+    set_needs_style_update(true);
 }
 
 }