Add a Background portal implementation for Linux

[Spaziale]

This would allow Thunderberd to run in the background so that it can check for new emails. Also, it is a common standard across multiple DEs, so not dependent on the individual implementation of Gnome, KDE or others.
With the official release of the flathub package, it would be very interesting if Thunderbird make use of portals

See https://docs.flatpak.org/en/latest/portal-api-reference.html#gdbus-org.freedesktop.portal.Background

[Jon]

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