qt: optimize playlist for desktop and not touch
Although having excessive padding makes it possible to interact with the delegate using touch control easier, it also causes the playlist to show fewer items at once.
VLC is primarily a desktop application, although we should care about touch support, we should not optimize the design for touch.
Reducing the vertical padding from 6dp to 3dp seems to be good compromise. It is still very much possible to use touch, but also looks okay for a desktop application.
Mozilla tried to optimize Firefox's design for touch with Proton, but it is disliked so much by the community that a new project got started just to fix the paddings [2]: We should not follow the same route.
