playlist: Move M3U-based file loading to happen after ctor runs.

This is another bug detected by LGTM
https://lgtm.com/projects/g/KDE/juk?mode=list

`loadFile()` itself calls at least four virtual methods. Rather than
trying to de-virtualize them as I did in the last fix, I decided to just
have the file load happen after the class is fully constructed, using
the Qt event loop.