Add high-level gsettings module

[jtojnar]

Now that #54150 was implemented in #234615 thanks to @linsui, we can implement a high-level GSettings module as a replacement for extraGSettingsOverrides.

I imagine something like

programs.gsettings = {
  checkAgainst = [ pkgs.gsettings-desktop-schemas ];
  settings = {
    "org/gnome/desktop/background" = {
       primary-color = "#000000";
    };
  };
};

which would turn into something like

programs.dconf = {
  enable = true;
  profiles.user.databases = [
    {
      settings = {
        "org/gnome/desktop/background" = {
           primary-color = "#000000";
        };
      };
    }
  ];
}

cc @NixOS/gnome @NixOS/pantheon

[nixos-discourse]

This issue has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/changing-gdm-gsettings-declaratively/49579/6

[ahydronous]

Instead of program.gsettings or program.dconf, wouldn't we rather move this into something like services.xserver.desktopManager.gnome with either .gsettings or .gsettingsOverride, or even .gsettings that would than have a bunch of attributes along with .gsettings.extraConfig for any deep config.

[jtojnar]

It is not really GNOME specific. And GSettings overrides are a specific thing (which we plan to actually remove) once this issue is resolved, so using that name would make it confusing.