XDG_CURRENT_DESKTOP not set will lead to xdg-desktop-portal errors

[cookiengineer]

For window managers that don't rely on a dbus session to set the XDG_CURRENT_DESKTOP environment variable, the xdg-desktop-portal throws the following error:

No such interface "org.freedesktop.portal.FileChooser" on object at path /org/freedesktop/portal/desktop

This error is caused by two stages:

1. The XDG_CURRENT_DESKTOP environment variable must be set.
2. A config file in /usr/share/xdg-desktop-portal/portals must match the currently set XDG_CURRENT_DESKTOP environment variable (e.g. UseIn=i3)

The missing environment variable is the cause of this problem for ly and i3 users. After looking at the code and successfully fixing this error (pull request incoming), it's a minor fix.

For Desktop Environments like GNOME, MATE, KDE, etc: These Desktop Environments set those environment variables via the dbus session in the background (which is started usually by the init.d implementation or systemd). That's probably why this error has not been reported before.

It would be awesome if this could get merged, because it took a while to understand/debug and was just a hazzle which can be avoided easily.