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/Page/EventHandler.h

@@ -48,6 +48,9 @@ public:
     bool handle_keydown(KeyCode, unsigned modifiers, u32 code_point);
 
 private:
+    bool focus_next_element();
+    bool focus_previous_element();
+
     LayoutDocument* layout_root();
     const LayoutDocument* layout_root() const;