Spider: Automatically reveal top cards

This matches basically all other Spider implementations I've played,
and makes playing much easier :)

I have left click-to-reveal in place, but mostly incase there is a
condition I've forgotten about.

Userland/Games/Spider/Game.h

@@ -67,6 +67,7 @@ private:
     void start_timer_if_necessary();
     void update_score(int delta);
     void draw_cards();
+    void ensure_top_card_is_visible(NonnullRefPtr<CardStack> stack);
     void detect_full_stacks();
     void detect_victory();