preferences.xiconfig is not read

I have this:

```bash
$ cat ~/.config/xi/preferences.xiconfig 
theme = "InspiredGitHub"
```

Setting theme manualy works:

`:t InspiredGitHub`

[logfile.txt](https://github.com/xi-frontend/xi-term/files/3672480/logfile.txt)

[logfile.rpc.txt](https://github.com/xi-frontend/xi-term/files/3672481/logfile.rpc.txt)
