printers: Use .boxed-list-separate in the printers list box

Each printer entry/row has several of elements such as texts and buttons. When there are lots of printers showing in the Printers panel (like in a corporate environment, for example), the printer list can look very cluttered.

To fix this problem, use the new .boxed-list-separate style class from libadwaita 1.6, which adds a gap between the rows from a list box.

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