Add flatpak support to UNIX Utility::setLaunchOnStartup

[FifthTundraG]

Discussion in flathub/com.nextcloud.desktopclient.nextcloud#225

This PR is the same as what the patch there is. For this to work flatpaks will need permission to acccess xdg-config/autostart on the host filesystem. If permission is not granted no autostart file is added.

[mgallien]

closing this PR in favor of a proper portal implementation
the exception needed to get something out of this patch has been denied making the change useless

[github-actions]

Hello there,
Thank you so much for taking the time and effort to create a pull request to our Nextcloud project.

We hope that the review process is going smooth and is helpful for you. We want to ensure your pull request is reviewed to your satisfaction. If you have a moment, our community management team would very much appreciate your feedback on your experience with this PR review process.

Your feedback is valuable to us as we continuously strive to improve our community developer experience. Please take a moment to complete our short survey by clicking on the following link: https://cloud.nextcloud.com/apps/forms/s/i9Ago4EQRZ7TWxjfmeEpPkf6

Thank you for contributing to Nextcloud and we hope to hear from you soon!

(If you believe you should not receive this message, you can add yourself to the blocklist.)