flatpak: .stackwallet inside the sandbox

[pierrelasse]

The .stackwallet dir should be marked as a persistent file (atleast that's what it's called in Flatseal) to make the folder be created inside the sandbox.

This logic can be reproduced by configuring it in Flatseal or using the flatpak command.
I've experienced no issues so far. ^

After this is done, the filesystem=home access can be removed.