added widget
added sf_widget
ui > stacked_widget:
using ui::widget as base type
ui > textbox:
derive from ui::widget instead of sf::Drawable
game > main_menu > site_home:
adapted to sf_widget
game > main_menu > site_credits:
adapted to sf_widget
game > main_menu > overlay
using data_store_type from common.hpp
activate_cam() > activate_current_cam()
game > main_menu
data_store_type