xdg-utils' xdg-open does not work in Toolbx

This is a continuation of #147, at the moment `xdg-open` is not working as intended,

```
$ /usr/bin/xdg-open somepdf.pdf

gio: file:somepdf.pdf: No application is registered as handling this file
```

```
/usr/bin/xdg-open "https://www.google.cl"

gio: https://www.google.cl: Operation not supported
```

The expected behaviour should be to open using the host default application, a possible fix is to create a `xdg-open` script in your path with contents
```shell
#!/bin/sh
${TOOLBOX_PATH:+flatpak-spawn --host} /usr/bin/xdg-open "$@"
```