Split now playing, the quiz buttons and song loading out of the quiz

NowPlaying, QuizToolbar and SongControls become their own scenes, the auto
repeat switch moves into the music player scene, and downloading songs moves
into a SongLoader node. The quiz only wires the parts together. 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 new part has its own tests.

tests/integration/test_layout_unchanged.gd

@@ -12,16 +12,24 @@ const BASELINE_PATH: String = "res://tests/data/layout_baseline.json"
 
 ## Nodes that moved from the main window into the Quiz scene, by their old name.
 const QUIZ_NODES: Array[String] = [
-	"Open", "FileDialog", "MusicListPanel", "VBoxContainer",
-	"MusicPlayer", "ResetPlaylistButton", "ResetPointsButton", "SettingsButton",
-	"SoundTestButton", "ShowAnswerButton", "NextButton", "RepeatSongCheckButton",
-	"DebugLabel", "SettingsWindow", "NewCharacterSelect", "WinnerWindow", "Log",
+	"Open", "FileDialog", "MusicListPanel", "DebugLabel", "SettingsWindow",
+	"NewCharacterSelect", "WinnerWindow", "Log",
 ]
 
-## Nodes that moved into the Quiz scene and were renamed there: old name to new name.
+## Nodes that moved into the Quiz scene under another path: old name to new path in
+## the quiz.
 const RENAMED_QUIZ_NODES: Dictionary = {
 	"Players": "PlayersPanel",
 	"ScrollContainer": "InspirationList",
+	"VBoxContainer": "NowPlaying",
+	"MusicPlayer": "MusicPlayer/PlayerBar",
+	"RepeatSongCheckButton": "MusicPlayer/RepeatSongCheckButton",
+	"ResetPlaylistButton": "QuizToolbar/ResetPlaylistButton",
+	"ResetPointsButton": "QuizToolbar/ResetPointsButton",
+	"SettingsButton": "QuizToolbar/SettingsButton",
+	"SoundTestButton": "QuizToolbar/SoundTestButton",
+	"ShowAnswerButton": "SongControls/ShowAnswerButton",
+	"NextButton": "SongControls/NextButton",
 }
 
 ## Nodes that moved from the main window into the TopBar scene, by their old name.