Use GtkFileChooserNative

[bbhtt]

Formiko doesn't seem to use GtkFileChooserNative that supports portal API but GtkFileChooser.

Portals allow to dynamically grant access to users rather than hard-coding arbitrary filesystem access for example for an application. The primary benefit here is tighter sandboxing; also better desktop integration.

[ondratu]

The problem not in GtkFileChooserNative, but with dynamical reading other files, just like style sheets or images, which can be used in documents, and Preview widget used them.

For this time, i don't have solution, but i leave this issue opened for the future.