Split the main window into TopBar and Quiz scenes

The quiz (song, playlist, players, music player and the popups opened from
it) moves into Quiz.tscn and the top panel into TopBar.tscn. The main window
keeps only the top bar, the quiz and the popups opened from the top bar, so
other views can be added next to the quiz later. Every node keeps its place
on screen.

Screenshots of every UI state before and after are pixel-identical, and a
layout test checks each node against a baseline taken before the split.

top_bar.gd

@@ -0,0 +1,22 @@
+class_name TopBar
+extends Control
+## The bar along the top of the main window, with the title and the buttons for things
+## that are not part of a quiz.
+
+signal search_pressed
+signal sync_pressed
+signal statistics_pressed
+signal shortcuts_pressed
+signal about_pressed
+signal qr_pressed
+
+@onready var sync_button: Button = $SyncButton
+
+
+func _ready() -> void:
+    $SearchButton.pressed.connect(search_pressed.emit)
+    sync_button.pressed.connect(sync_pressed.emit)
+    $StatisticsButton.pressed.connect(statistics_pressed.emit)
+    $ShortcutsButton.pressed.connect(shortcuts_pressed.emit)
+    $AboutButton.pressed.connect(about_pressed.emit)
+    $QRButton.pressed.connect(qr_pressed.emit)