Updated gui_screen_pregame_lobby_matchmaking

Implemented constructor/destructor
Implemented `create_bitmap_widget`
Implemented `get_gui_game_mode`
Implemented `get_lobby_header`
Implemented `get_lobby_title`
Implemented `get_start_button_name`
Implemented `handle_controller_input_message`
Added `handle_dialog_result`
Implemented `handle_list_item_chosen`
Implemented `initialize`
Added `is_lobby_in_live_mode`
Implemented `start_fade_during_countdown`/`stop_fade_during_countdown`
Added `update`
Implemented `update_nightmap`
Added `update_population`

Author: twist84