LibWeb: Separate layout tree rendering into phases

CSS defines a very specific paint order. This patch starts steering us
towards respecting that by introducing the PaintPhase enum with values:

- Background
- Border
- Foreground
- Overlay (internal overlays used by inspector)

Basically, to get the right visual result, we have to render the page
multiple times, going one phase at a time.

Libraries/LibWeb/Layout/LayoutWidget.h

@@ -35,8 +35,6 @@ public:
     LayoutWidget(const Element&, GUI::Widget&);
     virtual ~LayoutWidget() override;
 
-    virtual void render(RenderingContext&) override;
-
     GUI::Widget& widget() { return m_widget; }
     const GUI::Widget& widget() const { return m_widget; }