notification: Use notification desktop-id to select backend and notify

[3v1n0]

This is the continuation of https://github.com/flatpak/xdg-desktop-portal/pull/904

Notifications can provide a specific desktop-id, in such case we should just use the fdo notifications (to ensure that such info is consumed by the shell) if the provided ID is not a FDO application ID and pass such value to it, while when it's a valid application ID we can just use the gtk backend to have the full experience.

When desktop-id is not provided by the portal, it means it is matching the application-id, so we consider them the same.

[TingPing]

I feel like there should be some validation that the desktop-id is correct.

An application should not be able to report itself as a different application. So we should check that the desktop-id is prefixed by the app-id.

EDIT: This discussion probably should exist in xdg-desktop-portal instead of here.

[smcv]

Depends on flatpak/xdg-desktop-portal#904, converted to draft to avoid this getting merged before its required API is stable.

[TingPing]

https://github.com/flatpak/xdg-desktop-portal/pull/904 was closed and not merged so I believe this is no longer valid.