Move server calls to MusicServerClient and give players stable ids

Server requests move out of Settings into a new MusicServerClient
autoload with one function per endpoint. It sends requests through a
swappable HttpTransport so tests can fake the server, and it frees its
HTTPRequest nodes after use.

Players now have a UUID that never changes. Their position in
Settings.player_array is only used for the Alt+<number> keybinds.
Previously the position doubled as the id, so removing a player made
points and wins land on the wrong player, and could delete another
player's point from a song.

Also fixes:
- format_time showed 59.6 seconds as "00:60"
- searches containing regex characters such as "Q*bert" broke search
- SongObject and PlayerObject were Nodes that were never freed; they
  are now RefCounted
- search matching is split out of the UI code into find_matches()

winner_window.gd

@@ -17,10 +17,9 @@ func _ready() -> void:
 	visible = false
 
 
-func show_winner(winning_player_id: int) -> void:
+func show_winner(winner: PlayerObject) -> void:
 	visible = true
 	# Set winner info
-	var winner: PlayerObject = Settings.player_array[winning_player_id]
 	winner_label.text = winner.player_name + " won with " + str(winner.player_score) + \
 	" points!!"
 	winner_picture.texture = winner.character
@@ -41,7 +40,7 @@ func show_winner(winning_player_id: int) -> void:
 
 	var placing: int = 2
 	for player: PlayerObject in sorted_players:
-		if player.id == winning_player_id:
+		if player == winner:
 			continue  # Skip the winner, already shown above
 		var label: Label = Label.new()
 		label.text = str(placing) + ". " + player.player_name + " - " + \