Send remote file URIs as local FUSE URIs

Several people complain that they can't save files in remote drives (like a SMB mount) from containerized applications (like Firefox). After doing several tests, I found that it worked fine when using xdg-desktop-portal-gtk, but not when using xdg-desktop-portal-gnome.

The reason seems to be that xdg-desktop-portal-gtk translates the smb:// URIs (and other remote URIs like sftp://) into locally accessible file:// URIs (thanks to the FUSE backends available with Gvfs), but xdg-desktop-portal-gnome doesn't do that.

This patch fixes this by checking each URI obtained from the GtkFileChooser dialog, and if it isn't a "file://" one, it tries to translate it into its FUSE local URI.

Fix #45

Edited by Sergio Costas

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