Document GTK3 app dependencies

[VShell]

GTK3 apps require at least gnome3.gnome_icon_theme to be installed into either the system, default, or user profile. This should, at the least, be documented somewhere, and potentially should be installed by default in NixOS with X11 enabled, or a wrapper should be created so that all GTK3 apps in nixpkgs are launched with GTK environment variables pointing into this package.

Where the theme is searched for depends on $XDG_DATA_DIRS.

[lucabrunox]

There should be a common way to build gtk apps, yes. I haven't thought about that, but by now the pattern should be clear. Also, I think something can be cleaned up and wrapped better for apps to run on non-nixos.

[jtojnar]

Closing in favour of #16285