gnome.gnome-flashback: Add mkGnomeSession

Having this in environment.systemPackages shouldn't be worse than
adding as XDG_DATA_DIRS in gnome-session.

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

···
           map
             (wm:
               pkgs.gnome.gnome-flashback.mkSessionForWm {
-                inherit (wm) wmName wmLabel wmCommand enableGnomePanel;
+                inherit (wm) wmName wmLabel wmCommand;
               }
             ) flashbackWms;
 
···
         })
       ]
       # For /share/applications/${wmName}.desktop
-      ++ (map (wm: gnome-flashback.mkWmApplication { inherit (wm) wmName wmLabel wmCommand; }) flashbackWms);
+      ++ (map (wm: gnome-flashback.mkWmApplication { inherit (wm) wmName wmLabel wmCommand; }) flashbackWms)
+      # For /share/gnome-session/sessions/gnome-flashback-${wmName}.session
+      ++ (map (wm: gnome-flashback.mkGnomeSession { inherit (wm) wmName wmLabel enableGnomePanel; }) flashbackWms);
     })
 
     (mkIf serviceCfg.core-os-services.enable {

pkgs/desktops/gnome/misc/gnome-flashback/default.nix

···
           '';
         };
 
-      mkSessionForWm = { wmName, wmLabel, wmCommand, enableGnomePanel }:
+      mkGnomeSession = { wmName, wmLabel, enableGnomePanel }:
+        writeTextFile {
+          name = "gnome-flashback-${wmName}-gnome-session";
+          destination = "/share/gnome-session/sessions/gnome-flashback-${wmName}.session";
+          text = ''
+            [GNOME Session]
+            Name=GNOME Flashback (${wmLabel})
+            ${requiredComponents wmName enableGnomePanel}
+          '';
+        };
+
+      mkSessionForWm = { wmName, wmLabel, wmCommand }:
         let
-          gnomeSession = writeTextFile {
-            name = "gnome-flashback-${wmName}-gnome-session";
-            destination = "/share/gnome-session/sessions/gnome-flashback-${wmName}.session";
-            text = ''
-              [GNOME Session]
-              Name=GNOME Flashback (${wmLabel})
-              ${requiredComponents wmName enableGnomePanel}
-            '';
-          };
-
           executable = stdenv.mkDerivation {
             name = "gnome-flashback-${wmName}";
 
···
               makeWrapper ${gnome-session}/bin/gnome-session $out \
                 --add-flags "--session=gnome-flashback-${wmName} --builtin" \
                 --set-default XDG_CURRENT_DESKTOP 'GNOME-Flashback:GNOME' \
-                --prefix XDG_DATA_DIRS : '${lib.makeSearchPath "share" [ gnomeSession gnome-flashback ]}'
+                --prefix XDG_DATA_DIRS : '${lib.makeSearchPath "share" [ gnome-flashback ]}'
             '';
           };