replace underscores with spaces when parsing track file name

Fix for [Bug #1254534](https://bugs.launchpad.net/mixxx/+bug/1254534).
I found out that the metadata obtained by the ```TrackInfoObject::parseFilename()``` method was never used, because it was overridden by ```SoundSourceProxy::ParseHeader()```.

So, right now, in case the [```SoundSource::parseHeader() method```](https://github.com/mixxxdj/mixxx/blob/master/src/soundsourceproxy.cpp#L332) is not getting any metadata for Author, Title, or Type, it will use the values got by ```TrackInfoObject::parseFilename()```.