Enable UI scaling in the scenario and implement Linux menus

Patch from @x-qq
Addresses #195

Note: UI scaling for the scenario editor still needs some work,
but it's basically functional.

src/scenedit/scen.actions.hpp

@@ -8,7 +8,7 @@ void flash_rect(rectangle to_flash);
 void swap_terrain();
 void set_new_terrain(ter_num_t selected_terrain);
 void handle_keystroke(sf::Event event);
-void handle_scroll(sf::Event& event);
+void handle_scroll(const sf::Event& event);
 void get_wandering_monst();
 void get_town_info();
 void get_sign_resource();