Simplify app launching

The current code obtains the desktop file name to be lauched, resolves that to an absolute file path, passes it to a helper executable that then uses ApplicationLauncherJob

Instead we can get a KService from the desktop file name and pass that to ApplicationLauncherJob directly

In the Flatpak backend remove the fallback to flatpak_installation_launch, it should be safe to assume all Flatpaks can be launched through a desktop file