Applications menu for KDE apps in the Steam session

SteamOS only ships /etc/xdg/menus/plasma-applications.menu; the nested
desktop sets XDG_MENU_PREFIX=plasma-, the Steam session doesn't, so KDE
apps started there (Dolphin from the "+" menu) built an empty app
database: "no installed application can open" every file.

session.applicationsMenu (module applications-menu, on by default) links
~/.config/menus/applications.menu to Plasma's menu. Flake check: the link
is generated by default and absent when disabled.

template/home.nix

@@ -13,7 +13,8 @@
   # home.packages = with pkgs; [ htop ripgrep ];
 
   # Steam Frame fixes, see https://github.com/lhns/steam-frame-nix
-  # (session.portalFix and clipboardSync are on by default).
+  # (session.portalFix, session.applicationsMenu and clipboardSync are on
+  # by default).
   # steamFrame = {
   #   keyboard.layout = "de";               # XKB layout, Steam session
   #   keyboard.vr.extraKeys.enable = true;  # Esc/Ctrl/Alt/arrows in VR