xfsettingsd-gtk-settings-sync handles DPI incorrectly

I was wondering why when I reimplemented this in xfwl4 (sigh), with "correct" TitleCase->kebab-case code, I ended up with no text drawn at all, and the reason is that the units of gtk-xft-dpi are actually (dpi * 1024), so when setting it to a regular dpi value, you get impossible-to-see-at-all tiny text.

xfsettingsd-gtk-settings-sync doesn't do that conversion, but (because of #635 (closed)), this setting doesn't get set at all, so this issue never crops up.

This also raises the question of whether or not we should be setting gtk-xft-dpi on Wayland at all, as (I think) GTK figures out the DPI from the wl_output's reported physical size and display mode. I guess the intention here is to allow the user to customize the DPI? That feels kinda confusing on Wayland, to be honest... hm.