Launchers: neutral examples and made-up fixtures

Examples showed specific apps as if the repository installed or
configured them.

- Docs, README, template and option examples: the Signal Flatpak
  (org.signal.Signal, sgnl:// and signalcaptcha://) and a made-up
  org.example.App instead of the previous chat and remote-desktop apps;
  no install lines (the app is installed by the user, as noted).
- Portal fix: the file-dialog symptom phrased generically.
- Launcher check: made-up Flatpak entries org.example.Chat (Electron,
  --file-forwarding @@u %U @@, an action, an SSO scheme) and
  org.example.Remote (Qt, -qwindowtitle %c @@u %u @@, localized keys)
  replace the copied ones; Firefox, gedit and Jellyfin stay.

docs/session.md

@@ -51,8 +51,8 @@ session's `xdg-desktop-portal` at `/usr/share/xdg-desktop-portal/gamescope-porta
 which lacks `gamescope-portals.conf`: no backend, no OpenURI, so no app in
 the Steam session can open links. Its two backends (gamescope, holo) also
 have no FileChooser, so Flatpak apps fall back to a dialog inside their
-sandbox: e.g. Element's Attachments shows an empty file selector without
-your home dir.
+sandbox: a Flatpak app's file selector shows only its sandbox, not your
+home dir.
 
 **What you get:** a working OpenURI portal in the Steam session and, with
 `fileChooser`, KDE's file dialog there (a normal window in VR) with access