LibGUI+Browser: Add UrlBox class

This allows the address bar to "select all" when initially gaining focus
as Firefox and Chrome do. A future improvement on this would be for the
Widget class to mange and provide focus transition as part of the events
instead of the UrlBox class. Currently focus is updated before the event
is provided to the UrlBox class.

Userland/Libraries/LibGUI/TextEditor.h

@@ -142,6 +142,7 @@ public:
     void delete_previous_char();
     void delete_from_line_start_to_cursor();
     void select_all();
+    void select_current_line();
     virtual void undo();
     virtual void redo();