Show a graph of the game on the Winner popup

The Winner popup now shows every player's score after each song as a
step line, with the winning score marked, and three facts under it: the
longest streak, the longest standstill and the song most players scored
on. The data comes from the points each song already records, through
the new ScoreHistory. The popup grows to 1600x920 with the graph across
the full width, so a 60-song game with five players stays readable.

Reset points now starts a new game: it clears the points on the songs
and the graph starts after the song that was playing. Songs already on
the server at startup count as an earlier game.

The screenshot tour has a long five-player game, and only the Winner
popup differs from before.

quiz/song_loader.gd

@@ -33,6 +33,8 @@ func load_song_list() -> void:
 			new_song_object.has_played = true
 			Playlist.add_to_playlist(new_song_object)
 		Playlist.update_last_played_song()
+		# Songs already on the server were played before, so the game starts after them
+		Playlist.game_start_song = Playlist.song_object_array.size()
 		song_list_loaded.emit()
 		fetch_song_if_idle()
 	MusicServerClient.fetch_song_list(song_list_fetched, _retry_later.bind(load_song_list))