Instead of having to manually stub packages that should not be
downloaded we instead automatically stub all packages (except a small
list of whitelisted ones). Tests can re-introduce the real package by
using the `realPkgs` module argument.
This allows you to set a theme for Qt applications. For example, if you
want to use `adwaita-qt` theme to have uniform look between Gtk and Qt
applications, you can use it like this:
```nix
{
qt = {
enable = true;
platformTheme = "gnome";
style = {
name = "adwaita";
package = pkgs.adwaita-qt;
};
};
}
```
