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;
};
};
}
```
This change makes use of the `extend` function inside `lib` to inject
a new `hm` field containing the Home Manager library functions. This
simplifies use of the Home Manager library in the modules and reduces
the risk of accidental infinite recursion.
PR #994
