Spider: Add functionality to automatically move cards to valid stacks

This commit adds the possibility to use the secondary mouse button to
let the game move the selected card(s) to the next valid stack.

Userland/Games/Spider/Game.h

@@ -72,6 +72,7 @@ private:
     void ensure_top_card_is_visible(NonnullRefPtr<CardStack> stack);
     void detect_full_stacks();
     void detect_victory();
+    void move_focused_cards(CardStack& stack);
 
     ALWAYS_INLINE CardStack& stack(StackLocation location)
     {