nixos/flatpak: introduce guiPackages

This adds basically an indirection to systemPackages
to automatically install an interface for flatpak for their respective
environments. e.g if I enable pantheon and flatpak you'll get appcenter,
and on gnome you'll see gnome-software.

https://github.com/NixOS/nixpkgs/issues/99648#issuecomment-706691174

nixos/modules/services/desktops/flatpak.nix

@@ -15,6 +15,18 @@ in {
   options = {
     services.flatpak = {
       enable = mkEnableOption "flatpak";
+
+      guiPackages = mkOption {
+        internal = true;
+        type = types.listOf types.package;
+        default = [];
+        example = literalExample "[ pkgs.gnome3.gnome-software ]";
+        description = ''
+          Packages that provide an interface for flatpak
+          (like gnome-software) that will be automatically available
+          to all users when flatpak is enabled.
+        '';
+      };
     };
   };
 
@@ -28,7 +40,7 @@ in {
       }
     ];
 
-    environment.systemPackages = [ pkgs.flatpak ];
+    environment.systemPackages = [ pkgs.flatpak ] ++ cfg.guiPackages;
 
     services.dbus.packages = [ pkgs.flatpak ];
 

nixos/modules/services/x11/desktop-managers/gnome3.nix

@@ -207,6 +207,11 @@ in
 
        # If gnome3 is installed, build vim for gtk3 too.
       nixpkgs.config.vim.gui = "gtk3";
+
+      # Install gnome-software if flatpak is enabled
+      services.flatpak.guiPackages = [
+        pkgs.gnome3.gnome-software
+      ];
     })
 
     (mkIf flashbackEnabled {