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.

inspiration_list.gd

@@ -0,0 +1,67 @@
+class_name InspirationList
+extends ScrollContainer
+## The list of every soundtrack, shown for inspiration. It scrolls slowly up and down
+## by itself, at the speed set in the settings.
+
+## The soundtracks shown, in order.
+var soundtracks: Array = []
+
+# Scroll direction: 1 = down, -1 = up
+var scroll_direction: int = 1
+
+@onready
+var _rows: VBoxContainer = $VBoxContainer
+
+# Called when the node enters the scene tree for the first time.
+func _ready() -> void:
+    scroll_direction = 1
+
+# Called every frame. 'delta' is the elapsed time since the previous frame.
+func _process(delta: float) -> void:
+    var speed_pixels_per_second: float = float(Settings.inspiration_list_speed) * 60.0
+    var scroll_amount: float = speed_pixels_per_second * delta * float(scroll_direction)
+
+    var scroll_bar = get_v_scroll_bar()
+
+    # Don't scroll if list is empty or too short
+    if scroll_bar.max_value <= 0:
+        return
+
+    var current_pos = scroll_bar.value
+
+    if scroll_direction == 1:
+        # Scrolling down
+        if current_pos + scroll_bar.page < scroll_bar.max_value:
+            scroll_bar.value = min(current_pos + scroll_amount, scroll_bar.max_value - scroll_bar.page)
+        else:
+            # Reached bottom, reverse direction
+            scroll_direction = -1
+            scroll_bar.value = scroll_bar.max_value - scroll_bar.page
+    else:
+        # Scrolling up
+        if current_pos > 0:
+            scroll_bar.value = max(current_pos + scroll_amount, 0)
+        else:
+            # Reached top, reverse direction
+            scroll_direction = 1
+            scroll_bar.value = 0
+
+
+## Fetches every soundtrack from the server and shows them.
+func load_from_server() -> void:
+    print("load_from_server")
+    var populate_list: Callable = func populate_list(array) -> void:
+        if typeof(array) == TYPE_ARRAY:
+            show_soundtracks(array)
+        else:
+            print("Unexpected data")
+    MusicServerClient.fetch_all_soundtracks(populate_list)
+
+
+## Shows [param list] instead of the soundtracks shown now.
+func show_soundtracks(list: Array) -> void:
+    soundtracks = []
+    soundtracks.append_array(list)
+    Settings.delete_children(_rows)
+    for i: int in soundtracks.size():
+        _rows.add_child(ListRow.create(soundtracks[i], i, 20))