[Feature Request]: Flatpak Desktop file and appdata should live in the source repository

### Guidelines

- [X] I have searched the issue tracker for [open](https://github.com/FreeTubeApp/FreeTube/issues?q=is%3Aopen+is%3Aissue) and [closed](https://github.com/FreeTubeApp/FreeTube/issues?q=is%3Aissue+is%3Aclosed) issues that are similar to the feature request I want to file, without success.
- [X] I have searched the [documentation](https://docs.freetubeapp.io/) for information that matches the description of the feature request I want to file, without success.
- [X] This issue contains only one feature request.

### Problem Description

Desktop file and appdata should live in the source repository so that they can be kept in sync with upstream releases and people can contribute translations for it.

It should also be installed by the application's build process to the proper directories.

Currently, people are trying to send PRs to the flathub repo and closing them since the maintainer is inactive. It should live here, so that they can contribute.

https://github.com/flathub/io.freetubeapp.FreeTube/pull/87
https://github.com/flathub/io.freetubeapp.FreeTube/pull/83
https://github.com/flathub/io.freetubeapp.FreeTube/pull/70
https://github.com/flathub/io.freetubeapp.FreeTube/pull/76
https://github.com/flathub/io.freetubeapp.FreeTube/pull/58

Having a desktop file and appdata here is also a preferred prerequisite for creating distribution builds

### Proposed Solution

See above

### Alternatives Considered

None

### Issue Labels

new feature

### Additional Information

N/A