Sort the scripts and scenes into folders

main/ holds the main window and top bar, quiz/ the quiz and its parts,
player/ the music player, popups/ every popup and what they share, data/ the
songs, players, playlist and settings, server/ the MusicServer client,
common/ the list row and GameLog, and themes/ the themes and style boxes.

tests/integration/test_scene_split.gd

@@ -9,6 +9,17 @@ const MAIN_WINDOW_CHILDREN: Array[String] = [
 	"ShortcutsPopup", "StatisticsPopupPanel", "AboutPopup", "LogPopup",
 ]
 
+## The parts of the quiz and the scene each one comes from.
+const QUIZ_PART_SCENES: Dictionary = {
+	"MusicListPanel": "res://quiz/MusicListPanel.tscn",
+	"PlayersPanel": "res://quiz/PlayersPanel.tscn",
+	"InspirationList": "res://quiz/InspirationList.tscn",
+	"NowPlaying": "res://quiz/NowPlaying.tscn",
+	"QuizToolbar": "res://quiz/QuizToolbar.tscn",
+	"SongControls": "res://quiz/SongControls.tscn",
+	"MusicPlayer": "res://player/MusicPlayer.tscn",
+}
+
 ## The quiz buttons that are disabled while the server syncs.
 const LOCKED_BY_SYNC: Array[String] = [
 	"QuizToolbar/SoundTestButton", "QuizToolbar/ResetPlaylistButton",
@@ -57,10 +68,9 @@ func test_top_bar_is_drawn_below_the_quiz() -> void:
 
 
 func test_quiz_parts_are_their_own_scenes() -> void:
-	for node_name: String in ["MusicListPanel", "PlayersPanel", "InspirationList",
-			"NowPlaying", "QuizToolbar", "SongControls", "MusicPlayer"]:
+	for node_name: String in QUIZ_PART_SCENES:
 		assert_str(_quiz.get_node(node_name).scene_file_path) \
-				.is_equal("res://%s.tscn" % node_name)
+				.is_equal(QUIZ_PART_SCENES[node_name])
 
 
 func test_quiz_downloads_songs_with_a_song_loader() -> void: