set `xdg-open` to be a failsafe for Nix Desktops

- remove flux box detection as it uses xdg-open

Fixes #70
if the path is valid and the schema is invalid, open the file

Fixes #65
cleanup BROWSER env var after tests
be consistent about getting the app from the string
revert unneeded change
update fixme
making children order dependent
adding a null object style default environment