Update MainWindow.vala to enable xdg-open in BTRFS mode

[ferreol-fr]

Helo,
I don't see any reason not to try using xdg-open with BTRFS mode.
(If there is one, it would be helpful to add it as a comment.)

[ygerlach]

The variable doesn't say if we use xdg or not. It says if we try xdg before we try the filemanagers or after.
This variable is just responsible for ordering. Why the order was chosen to be like this, i don't know.

This issue was the reason this option was added at some point. Maybe we can drop it now completely?
I don't see any reason, why exo_open_folder should check xdg_open sometimes first and sometimes last.
teejee2008/timeshift#186