Add --files= option to setup.py build_ui

Usage example:
```bash
$> python setup.py build_ui --files=options,options_interface
```

Those are equivalent:
```bash
python setup.py build_ui --files=ui_options
python setup.py build_ui --files=ui/options.ui
python setup.py build_ui --files=picard/ui/ui_options.py
python setup.py build_ui --files=options
```

Related to http://tickets.musicbrainz.org/browse/PICARD-566