Add AppStream metadata

Most of the Software Centers need AppStream metadata to find application properly. Currently this metadata file is generated out of the desktop file. But this is deprecated and lacking a lot metadata (see e.g. author field and copyright).


It would be helpful if nextcloud desktop itself would ship a appstream metadata file.

## MetaInfo Localization

AppStream MetaInfo files are recognized by `xgettext`, so you just need to add com.nextcloud.desktopclient.nextcloud to your POTFILES.in to generate templates for all translatable elements in the file. When building the software, you can then instruct your build system to merge in the translations into the new file before installing the translated file as `/usr/share/metainfo/com.nextcloud.desktopclient.nextcloud.metainfo.xml`.

A first start of the appstream metadata file. See :

```xml

com.nextcloud.desktopclient.nextcloud

CC0-1.0
GPL-2.0-or-later

Nextcloud

Nextcloud Desktop
Nextcloud desktop synchronization client

The Nextcloud desktop app lets you always have your latest files wherever you are.
Just specify one or more folders on the local machine to and a server to synchronize to.
You can configure more computers to synchronize to the same server and any change to
the files on one computer will silently and reliably flow across to every other.

com.nextcloud.desktopclient.nextcloud.desktop

Nextcloud Desktop under Windows
https://nextcloud.com/c/uploads/2022/12/nextcloud-windows-desktop-integration.jpg


Office
Utility

https://nextcloud.com
https://github.com/nextcloud/desktop/issues

nextcloud-desktop

```