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/HTML/HTMLAnchorElement.h

@@ -39,6 +39,8 @@ public:
 
     String href() const { return attribute(HTML::AttributeNames::href); }
     String target() const { return attribute(HTML::AttributeNames::target); }
+
+    virtual bool is_focusable() const override { return has_attribute(HTML::AttributeNames::href); }
 };
 
 }