Use Flatpak portals for notifications

[Jacalz]

Checklist

• I have searched the issue tracker for open issues that relate to the same feature, before opening a new one.
• This issue only relates to a single feature. I will open new issues for any other features.

Is your feature request related to a problem?

Fyne applications, like https://beta.flathub.org/apps/details/io.github.jacalz.rymdport, currently have to enable --talk-name=org.freedesktop.Notifications to support notifications over Dbus. It would be better to instead use the Flatpak portal for this as it would avoid having to enable more than permissions and also would allow the Gnome/Plasma applications settings to turn on and off notifications for the application.

Is it possible to construct a solution with the existing API?

Yes. We should just send the notification in a different way that what we currently are doing.

Describe the solution you'd like to see.

I don't know if this has been done using Go before but seems, per https://libportal.org/libportal.html, like this should be doable over Dbus. Worst case, we need to create an optional CGo wrapper around https://github.com/flatpak/libportal.

Using go build -tags flatpak might be required to solve this.