Added functionality of playlist-related buttons

Took a little while but now instead of using the playlistWidget's list because of some complications with that list, we have another QStringList list which we easily load to the playlist list with playlist->addItems(list);
Shuffling is done with std::random_shuffle from std::algorithm, it just shuffles the existing list, this should be added as a simple button at the bottom of the playlist like the rest of them.