Integrate with GNOME Online Accounts Single Framework

GNOME has a feature for managing online accounts in one place ( [GnomeOnlineAccounts](https://wiki.gnome.org/Projects/GnomeOnlineAccounts) | `GOA`).

In it's current state Nextcloud is supported and it can read calendar, contacts and documents (as in providing a quick mount option in the file explorer): [https://wiki.gnome.org/Projects/GnomeOnlineAccounts/Providers](https://wiki.gnome.org/Projects/GnomeOnlineAccounts/Providers)

This way account credentials can be shared and are configured and managed in one place where they can be [requested](https://developer.gnome.org/goa/stable/ch01s03.html) by an application. The communication is done via `D-Bus`.

Currently I have to configure my accounts in two places, the `nextcloud-client` and in `GOA`, to get a Nextcloud server tightly integrated into my system and while the passwords end up in the same place (`gnome-keyring`) it'd be more user friendly to share this from a common ground instead of having to configure this twice for each Nextcloud account.