Feat: Flatpak support

[bryce-hoehn]

Addresses #211, #517, #619, and partially addresses #244.

Based on the manifest created by NeoTheFox, with an additional Github Actions workflow for building the flatpak. Currently the build just uploads as a zip into the workflow artifacts. It shouldn't be too much extra work to add a step for publishing to Flathub, but it would need to come from the official team.

[bryce-hoehn]

It should now be ready for review. I've added github actions to automatically update & publish the flatpak packages alongside releases. If I understand correctly, all that needs done now is creating the initial PR to Flathub, which needs to be done by the OpenCloud team. Documentation: https://docs.flathub.org/docs/for-app-authors/submission#submission-pr

In compliance with Flathub's application ID requirements, I also went ahead and replaced eu.opencloud-eu.desktop with eu.opencloud-eu.OpenCloudDesktop throughout the application. This might be a breaking change for existing installs, but could be mitigated by selectively applying the name only on flatpak builds.

Let me know if there is anything you'd like me to change!

[bryce-hoehn]

I've noticed a few minor things that might need to be addressed before approval.

1. Currently the workflow updates the manifest after a release is triggered. This technically means the source code bundle in each release would still have the manifest from the previous release. Not sure if this really makes a difference for Flathub.
2. If you wanted to commit to changing the app id throughout all platforms, we could probably implement a migration script to move existing directories with the old app id.