LibWeb: Add Painting::Box and move things from Layout::Box into it

The "paintable" state in Layout::Box was actually not safe to access
until after layout had been performed.

As a first step towards making this harder to mess up accidentally,
this patch moves painting information from Layout::Box to a new class:
Painting::Box. Every layout can have a corresponding paint box, and
it holds the final used metrics determined by layout.

The paint box is created and populated by FormattingState::commit().

I've also added DOM::Node::paint_box() as a convenient way to access
the paint box (if available) of a given DOM node.

Going forward, I believe this will allow us to better separate data
that belongs to layout vs painting, and also open up opportunities
for naturally invalidating caches in the paint box (since it's
reconstituted by every layout.)

Userland/Libraries/LibWeb/DOM/Node.cpp

@@ -1010,4 +1010,13 @@ size_t Node::length() const
     return child_count();
 }
 
+Painting::Box const* Node::paint_box() const
+{
+    if (!layout_node())
+        return nullptr;
+    if (!layout_node()->is_box())
+        return nullptr;
+    return static_cast<Layout::Box const&>(*layout_node()).m_paint_box;
+}
+
 }