Follow the GDScript style guide in every script

- Types on every parameter, return value and variable, and snake_case names
  (the player name limit is the constant MAX_NAME_LENGTH).
- Spelling fixes: *_triggered, character_selected, separator, caught_errors.
  The server's catched_errors key stays as the server sends it.
- Signals are connected with signal.connect() instead of by name.
- Code is ordered signals, constants, exported vars, vars, @onready vars,
  functions, with @onready on one line and two blank lines between functions.
- Every script starts with a ## description. Template comments, old LOC notes
  and commented-out code are gone, apart from the local play code.
- Lines are at most 100 characters, apart from the local play path.
- The screenshot tour turns off the blinking text cursor so pictures match.

music_list_panel.gd

@@ -3,12 +3,12 @@ extends PanelContainer
 ## The list of songs in the playlist, one row per song.
 
 ## Emitted when the row of song number [param song_no] is clicked.
+
 signal song_clicked(song_no: int)
 
 var _song_list_object: PackedScene = preload("res://SongRow.tscn")
 
-@onready
-var _rows: VBoxContainer = %MusicList
+@onready var _rows: VBoxContainer = %MusicList
 
 
 ## Shows the songs in the playlist again, so it shows their latest state.