v0.47.1: the no-art panel sits on the window, shaded like the visualizer

With no art, the sidebar's art widget moves out of the playlist list's
frame onto the window below it, so the panel is set into the gray like the
visualizer rather than into a white box. A tall debossed panel is now
shaded along its top and bottom exactly as the 62 px visualizer is, flat
in the visualizer's color between, instead of one gradient stretched over
its whole height, which read as gray. The scrollbar thumb is shaded the
same way, like a button.

tests/test_round69.py

@@ -52,8 +52,9 @@ def test_library_button_spans_the_playlist_list(qapp, tmp_path):
     window = _window(qapp, tmp_path)
     header = window.sidebar.header
     lists = window.sidebar.list_area
-    # The frame around the playlists takes in the album art too.
-    assert window.sidebar.art.parentWidget() is lists
+    # With nothing playing the art is the empty panel, below the frame on
+    # the window (Round 80); the frame takes in real art (test_round80).
+    assert window.sidebar.art.parentWidget() is window.sidebar
     assert header.parentWidget() is not window.sidebar
     assert _gx(header) == _gx(lists) > _gx(window)
     assert header.width() == lists.width()