Ok, I have arrived at something, I still have to understand the
details but many things are correctly scaled, including widget images.

Icons in the fringe seems to be ok but yet:
1. So checkboxes are weirdly clipped, even if correctly scaled.
2. Not sure about this, but I believe there is a slight vertical misalignment.
3. Default fringe sizes are too small for hidpi screen, they should
change according to screen dpi.

I'm attaching a diff so that you can have an idea of what I am
changing and probably improve on that.