gnome.gnome-flashback: Add mkWmApplication

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

···
         (gnome-panel-with-modules.override {
           panelModulePackages = cfg.flashback.panelModulePackages;
         })
-      ];
+      ]
+      # For /share/applications/${wmName}.desktop
+      ++ (map (wm: gnome-flashback.mkWmApplication { inherit (wm) wmName wmLabel wmCommand; }) flashbackWms);
     })
 
     (mkIf serviceCfg.core-os-services.enable {

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

···
         versionPolicy = "odd-unstable";
       };
 
+      mkWmApplication = { wmName, wmLabel, wmCommand }:
+        writeTextFile {
+          name = "gnome-flashback-${wmName}-wm";
+          destination = "/share/applications/${wmName}.desktop";
+          text = ''
+            [Desktop Entry]
+            Type=Application
+            Encoding=UTF-8
+            Name=${wmLabel}
+            Exec=${wmCommand}
+            NoDisplay=true
+            X-GNOME-WMName=${wmLabel}
+            X-GNOME-Autostart-Phase=WindowManager
+            X-GNOME-Provides=windowmanager
+            X-GNOME-Autostart-Notify=false
+          '';
+        };
+
       mkSessionForWm = { wmName, wmLabel, wmCommand, enableGnomePanel }:
         let
-          wmApplication = writeTextFile {
-            name = "gnome-flashback-${wmName}-wm";
-            destination = "/share/applications/${wmName}.desktop";
-            text = ''
-              [Desktop Entry]
-              Type=Application
-              Encoding=UTF-8
-              Name=${wmLabel}
-              Exec=${wmCommand}
-              NoDisplay=true
-              X-GNOME-WMName=${wmLabel}
-              X-GNOME-Autostart-Phase=WindowManager
-              X-GNOME-Provides=windowmanager
-              X-GNOME-Autostart-Notify=false
-            '';
-          };
-
           gnomeSession = writeTextFile {
             name = "gnome-flashback-${wmName}-gnome-session";
             destination = "/share/gnome-session/sessions/gnome-flashback-${wmName}.session";
···
               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 : '${lib.makeSearchPath "share" [ wmApplication gnomeSession gnome-flashback ]}'
+                --prefix XDG_DATA_DIRS : '${lib.makeSearchPath "share" [ gnomeSession gnome-flashback ]}'
             '';
           };