Add API to register app ID

[jadahl]

Meant to be used by host apps to more reliably get an app ID. See flatpak/xdg-desktop-portal#1521.

[GeorgesStavracas]

LGTM, the only thing I think is missing is the documentation blocks for introspection.

[jadahl]

Tweaked the qt examples a bit. The qt5 one now plumbs the app Id via QApplication, but couldn't do that in qt6 - I had to register the app ID before doing QApplication a(argc, argv); since constructing the QApplication already started to talk to the portal (I'm guessing for settings or something).

[ilya-fedin]

The qt5 one now plumbs the app Id via QApplication, but couldn't do that in qt6 - I had to register the app ID before doing QApplication a(argc, argv); since constructing the QApplication already started to talk to the portal (I'm guessing for settings or something).

desktopFileName and setDesktopFileName are static methods of QGuiApplication (which QApplication inherits from), you can use them before constructing QApplication just fine

[smcv]

desktopFileName and setDesktopFileName are static methods of QGuiApplication (which QApplication inherits from), you can use them before constructing QApplication just fine

Does this mean it would be more correct to call them as QGuiApplication::setDesktopFileName(APP_ID) and QGuiApplication::desktopFileName(), rather than app->setDesktopFileName(APP_ID) and app->desktopFileName()?

[ilya-fedin]

Does this mean it would be more correct to call them as QGuiApplication::setDesktopFileName(APP_ID) and QGuiApplication::desktopFileName(), rather than app->setDesktopFileName(APP_ID) and app->desktopFileName()?

Exactly

[smcv]

Please check whether the Qt code paths can be improved based on what @ilya-fedin said