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.

data/playlist.gd

@@ -11,6 +11,9 @@ var first_song_downloaded: bool = false
 var first_song_played: bool = false
 var number_of_tracks_to_preload: int = 1
 var is_song_downloading: bool = false
+## The first song of the current game. Songs before it belong to earlier games, which
+## ended with Reset points.
+var game_start_song: int = 0
 
 
 func get_next_song() -> SongObject:
@@ -68,6 +71,7 @@ func get_playlist() -> Array[SongObject]:
 func reset_playlist() -> void:
 	currently_playing_song = -1
 	latest_played_song = 0
+	game_start_song = 0
 	clear_song_list()
 
 
@@ -100,6 +104,12 @@ func remove_point(player_id: String) -> void:
 	song_object_array[currently_playing_song].remove_point(player_id)
 
 
+## Removes every player's points from every song, for a new game.
+func clear_points() -> void:
+	for song_object: SongObject in song_object_array:
+		song_object.players_given_point.clear()
+
+
 ## Removes a player's points from every song.
 func remove_player_from_list(player_id: String) -> void:
 	for song_object: SongObject in song_object_array: