Use XDG Desktop Portals to decrease Flatpak permissions

[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?

The Flatpak package on Flathub currently has access to the entire home directory (read only though) and then the downloads folder. Given that I implemented support for desktop portals upstream in Fyne, we should start looking at using them now.

Describe the solution you'd like to see.

Rework file save to open a dialog asking the user where to save each file. After that we should update the Flatpak package to don’t have any file permissions set.