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()

new_character_select.gd

@@ -45,7 +45,7 @@ func load_characters() -> void:
 				character_box.size = Vector2(235, 145)
 				character_box.set_character_name(character_name)
 				character_box.connect("new_character_selected", _on_character_selected)
-	Settings.make_request2("/characters", fetch_character_list, true)
+	MusicServerClient.fetch_character_list(fetch_character_list)
 
 func show_grid(player_name: String) -> void:
 	print("show_grid")