Use portals for file access on Linux

### Your use case

#### What would you like to do?

Use XDG portals for file access, as GTK and Qt do

#### Why would you like to do it?

Provides a native interface and unbreaks the Flatpak package.

#### How would you like to achieve it?

Use the XDG portal API.

### Have you considered any alternatives?

None

### Additional context

Portals are the modern way to access files on desktop Linux. They are supported by all major desktop environments.