Make the music list, player list and inspiration list their own scenes

MusicListPanel, PlayersPanel (was Players) and InspirationList (was
ScrollContainer) each get their own scene and script. They report what
happens through signals, and the quiz wires them to the playlist, the
music player, the log and the popups. Every node keeps its place on screen.

Screenshots of every UI state are pixel-identical to before, the layout
test still matches the original baseline, and each part is tested on its
own as well as through the quiz.

tests/fakes/main_window_test_suite.gd

@@ -77,16 +77,16 @@ func _play_first_song() -> void:
 ## Adds a player through the name field, the same way the user does.
 func _add_player(player_name: String) -> PlayerObject:
     var name_field: TextEdit = _quiz.get_node(
-            "Players/VBoxContainer/AddPlayerContainer/PlayerNameField")
+            "PlayersPanel/VBoxContainer/AddPlayerContainer/PlayerNameField")
     name_field.text = player_name
-    _quiz.add_player()
+    _quiz.players_panel.add_player()
     return Settings.player_array.back() if !Settings.player_array.is_empty() else null
 
 
 ## Returns the row in the player list that shows [param player_object].
 func _player_row(player_object: PlayerObject) -> Player:
     var rows: Node = _quiz.get_node(
-            "Players/VBoxContainer/ScrollContainer/PlayersListVBoxContainer")
+            "PlayersPanel/VBoxContainer/ScrollContainer/PlayersListVBoxContainer")
     for row: Player in rows.get_children():
         if row.player_object == player_object:
             return row