Only show delete and save playlist button when item is selected

Fixes #500

src/playlist/playlistlistview.cpp

@@ -56,3 +56,11 @@ void PlaylistListView::paintEvent(QPaintEvent *event) {
     AutoExpandingTreeView::paintEvent(event);
   }
 }
+
+bool PlaylistListView::ItemsSelected() const {
+  return selectionModel()->selectedRows().count() > 0;
+}
+
+void PlaylistListView::selectionChanged(const QItemSelection&, const QItemSelection&) {
+  emit ItemsSelectedChanged(selectionModel()->selectedRows().count() > 0);
+}