Fix crash in Lyrics plugin when there are no results
Kill PlaylistPopulator and PlaylistWriter threads

We now use custom runnable in PlaylistModel instead, which gives us more
control over metadata loading.

First step towards killing TaskManager
Fix M3U::wirteToFile() not writing any data to files
Move MetaData outside Player scope and make it an implicitely shared class

This allows us to share the structure accross TepSonic more effectively