fix(ui): B never quits (Steam sends Esc for it), Ctrl+Q does; started from Steam, the hints show the Steam Controller's buttons

ui/steamify-ui

@@ -373,6 +373,9 @@ def main():
     engine = QQmlApplicationEngine()
     engine.setInitialProperties({"backend": backend, "gamepad": pad,
                                  "fullscreen": fullscreen,
+                                 # Started from Steam: its keys are a Steam
+                                 # Controller's buttons (desktop layout).
+                                 "fromSteam": bool(os.environ.get("SteamGameId")),
                                  "iconUrl": QUrl.fromLocalFile(os.path.realpath(icon)).toString()})
     engine.load(os.path.join(HERE, "qml", "Main.qml"))
     if not engine.rootObjects():