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/DOM/Element.cpp

@@ -295,4 +295,9 @@ String Element::inner_html() const
     return builder.to_string();
 }
 
+bool Element::is_focused() const
+{
+    return document().focused_element() == this;
+}
+
 }