[Development] High-dpi Qt best practices

[Bache-Wiig Jens]

> 
> The only thing is about QPen. should we draw lines twice as large?  that is 
> bascically what QPen::isCosmetic is for

Absolutely. The problem is that for some reason we have cosmetic as the default.
In practice people hardly use cosmetic pens for what it was designed for, it is imply treated as 1px thick pen by pretty much anything QPainter is used for. And even when cosmetic pens are used intentionally, 1 pixel thick lines simply don't look proper on a Retina display. I would definitely +2 this idea. 

Jens