Refactor the game event loop and add menus to the Linux build

Patch from @x-qq
Addresses #195

src/game/boe.graphics.hpp

@@ -51,7 +51,9 @@ bool is_nature(short i, short j, unsigned short ground_t);
 void put_dialog_graphic(short graphic_num,short spec_g,rectangle draw_rect);
 void draw_startup_stats();
 void draw_trim(short q,short r,short which_trim,ter_num_t ground_ter);
+sf::FloatRect compute_viewport (sf::RenderWindow const &, int mode, float ui_scale, float width, float height);
 
 void draw_startup_anim(bool advance);
 
+
 #endif