Scroll wheel support

- All areas with scroll wheels can be scrolled using the scroll wheel if the mouse is in the area
- In addition, the terrain view is scrollable with the scroll wheel (hold Control for horizontal scrolling)
- The above applies in the game too, whenever the scroll arrows are visible

src/boe.actions.h

@@ -14,6 +14,7 @@ bool someone_awake();
 void handle_menu_spell(short spell_picked,short spell_type) ;
 void initiate_outdoor_combat(short i);
 bool handle_keystroke(sf::Event& event);
+bool handle_scroll(sf::Event& event);
 void do_load();
 void post_load();
 void do_save(short mode);