set Gtk/IconSizes based on (effective) font size
Submitted by Allison (desrt)
Setting aside all the arguments about if it makes sense to automatically change the size of various bits of the UI based on DPI, one thing that does make sense is that the icon in a menuitem should probably be approximately the same height as the line of text beside it.
For that reason, if the user is increasing the font size, probably we should increase icon size.
We can do this via the Gtk/IconSizes XSETTING, which is not currently set by gnome-settings-daemon.