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/integration/test_main_window.gd

@@ -240,12 +240,3 @@ func test_log_adds_one_row_per_point_with_padded_time() -> void:
     var time_pattern: RegEx = RegEx.create_from_string("^\\d\\d:\\d\\d:\\d\\d Crono got a point$")
     assert_object(time_pattern.search((log_box.get_child(0) as Label).text)).is_not_null()
     assert_str((log_box.get_child(1) as Label).text).ends_with("Crono lost a point")
-
-
-func test_inspiration_list_alternates_row_backgrounds() -> void:
-    var rows: Array[Node] = _quiz.get_node("ScrollContainer/VBoxContainer").get_children()
-
-    assert_int(rows.size()).is_equal(2)
-    assert_str((rows[0] as ListRow).text).is_equal("Chrono Trigger")
-    assert_that((rows[0] as ListRow).get_theme_stylebox("panel")) \
-            .is_not_equal((rows[1] as ListRow).get_theme_stylebox("panel"))