selection_menu: Move it to it's dedicated class

includes/context.hpp

@@ -18,12 +18,14 @@ class Render;
 
 namespace gol {
 class SettingsMenu;
+class SelectionMenu;
 
 typedef struct ctx {
   std::shared_ptr<World> world = nullptr;
   std::shared_ptr<Rules> rules = nullptr;
   std::shared_ptr<Render> render = nullptr;
   std::shared_ptr<SettingsMenu> settings_menu = nullptr;
+  std::shared_ptr<SelectionMenu> selection_menu = nullptr;
   nlohmann::json config_json;
 } ctx;