LibGUI: Improve automatic focus guessing somewhat

When opening a new window, we'll now try to find a suitable widget for
initial focus by picking the first available mouse-focusable one.

Whenever you press the tab key in a window with no focused widget,
we'll attempt to find a keyboard-focusable widget and give it focus.

This should make all applications keyboard-interactive immediately
without having to manually place focus with the mouse.

Libraries/LibGUI/Window.h

@@ -204,6 +204,7 @@ private:
     virtual bool is_window() const override final { return true; }
 
     void update_cursor();
+    void focus_a_widget_if_possible(FocusSource);
 
     void handle_drop_event(DropEvent&);
     void handle_mouse_event(MouseEvent&);