Support the background apps portal (`org.freedesktop.portal.Background`)

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## Feature request

People who use this app (instead of GNOME Online Accounts' built-in NextCloud feature, or people on other modern sandboxed & portalized Wayland-based Linux desktops) sometimes have the expectation that it would show some indication that it's running in the background on Pantheon, GNOME, or other Linux desktops that don't provide a legacy tray icon widget. This app, [along with Steam](https://github.com/ValveSoftware/steam-for-linux/issues/9970), is pretty much the only thing I keep hearing complaints about on this front (personally, I have over a hundred GUI apps installed and none of them have this problem).

To help solve this, this application should support the [FreeDesktop background apps portal](https://flatpak.github.io/xdg-desktop-portal/docs/doc-org.freedesktop.portal.Background.html) (`org.freedesktop.portal.Background`).

This would allow the NextCloud desktop sync app to better let the operating system (and user) know when it's running in the background, and desktops to potentially better integrate it, now and in the future. Additionally, an app relying on tray icons isn't the best UX. They're inconsistent, buggy, not compatible with a sandbox, and barely function as-is (as @orowith2os said in the Steam ticket).

Historical context:
* https://blogs.gnome.org/aday/2017/08/31/status-icons-and-gnome/
* https://blog.tingping.se/2019/09/07/how-to-design-a-modern-status-icon.html
* Future UX in GNOME: https://gitlab.gnome.org/Teams/Design/os-mockups/-/work_items/191