The sidebar's art square, when a track has no art, is now the list's own
background with a rounded panel set into it, debossed like the visualizer
and the now-playing panel. paint_deboss takes its rims in pixels when
asked, so a panel as tall as the sidebar is recessed as much as a short
one rather than fading over 16 px.
The app style now paints the scrollbars itself: the trough runs the full
length with no arrow buttons, and the thumb gets the buttons' 4 px corners
and emboss, with rims as tall in pixels as a button's however long the
thumb. Bar thickness moved from QSS to the style's pixel metric, since any
QSS rule on a scrollbar part makes Qt draw the whole bar, arrows included.
trav