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/ui_tour.gd

@@ -179,11 +179,14 @@ func _node(node_name: String) -> Node:
     return null
 
 
-## Calls [param method] on the main window, or on its Quiz scene if that has it.
+## Calls [param method] on the players panel, the quiz or the main window, whichever
+## has it first.
 func _call(method: String, args: Array = []) -> Variant:
-    var quiz: Node = _main.get_node_or_null("Quiz")
-    var target: Node = quiz if quiz != null && quiz.has_method(method) else _main
-    return target.callv(method, args)
+    for path: String in ["Quiz/PlayersPanel", "Quiz"]:
+        var target: Node = _main.get_node_or_null(path)
+        if target != null && target.has_method(method):
+            return target.callv(method, args)
+    return _main.callv(method, args)
 
 
 func _server() -> FakeHttpTransport:
@@ -196,13 +199,13 @@ func _settle() -> void:
 
 
 func _add_player(player_name: String) -> void:
-    (_node("Players").get_node("VBoxContainer/AddPlayerContainer/PlayerNameField") \
+    (_node("PlayersPanel").get_node("VBoxContainer/AddPlayerContainer/PlayerNameField") \
             as TextEdit).text = player_name
     _call("add_player")
 
 
 func _player_row(index: int) -> Player:
-    return _node("Players").get_node(
+    return _node("PlayersPanel").get_node(
             "VBoxContainer/ScrollContainer/PlayersListVBoxContainer").get_child(index)