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.

README.md

@@ -218,9 +218,9 @@ these two files ([`template/`](template), with more comments):
     firefox.disableAv1 = true;
     firefox.defaultBrowser = true;
     jellyfin.hardwareDecoding.enable = true;  # install the Flatpak yourself
-    launchers."im.riot.Riot" = {              # Element: logins in both sessions
+    launchers."org.signal.Signal" = {         # logins in both sessions; install the Flatpak yourself
       keyring = { enable = true; electron = true; };
-      defaultFor = [ "x-scheme-handler/element" "x-scheme-handler/io.element.desktop" ];
+      defaultFor = [ "x-scheme-handler/sgnl" "x-scheme-handler/signalcaptcha" ];
     };
     docker.enable = true;                     # rootless
   };