disable previous and next button

if there is no corresponding media; set play/pause symbol via model
property instead of via qml.

LenaPi/controllers/MusicController.cpp

@@ -19,6 +19,8 @@ MusicController::MusicController(QObject *parent) : QObject(parent)
     connect(mModel, &MusicModel::navigateTo, [this](NavigationItemModel*){
         if(mModel->isPlaying()) mModel->playPause();
     });
+
+    connect(mPlayer, SIGNAL(nextItemSet(VlcMedia*)), mModel, SLOT(onNextMediaSet(VlcMedia*)));
 }
 
 MusicController::~MusicController()