LibWeb: Some improvements to absolute positioning

Absolutely positioned blocks now register themselves with their
containing block (and note that the containing block of an absolutely
positioned box is the nearest non-statically positioned block ancestor
or the ICB as fallback.)

Containing blocks then drive the layout of their tracked absolutely
positioned descendants as a separate layout pass.

This is very far from perfect but the general direction seems good.

Libraries/LibWeb/Layout/BoxModelMetrics.h

@@ -54,6 +54,8 @@ public:
     };
 
     PixelBox full_margin(const LayoutNode&) const;
+    PixelBox padding_box(const LayoutNode&) const;
+    PixelBox border_box(const LayoutNode&) const;
 
 private:
     LengthBox m_margin;