qml: make playlist listview's background opaque in small screen mode

Playqueue pane's background uses a hack to block what is shown beneath but instead makes window background to pass through. This hack is used because play queue is hierarchically placed in a position where there is background that is not fully transparent.

This conflicts with the "small screen" view, since the stack view does not get unusably small but rather blows up into the area used by the play queue. This constitutes a problem, because play queue effectively prevents rendering the part of the stack view that overlaps with its size.

Request review @chub.