LibWeb: Allow focusing individual (focusable) elements with Tab key

You can now cycle through focusable elements (currently only hyperlinks
are focusable) with the Tab key.

The focus outline is rendered in a new FocusOutline paint phase.

Libraries/LibWeb/Layout/LayoutNode.h

@@ -103,6 +103,7 @@ public:
         Background,
         Border,
         Foreground,
+        FocusOutline,
         Overlay,
     };
     virtual void paint(PaintContext&, PaintPhase);