Fix remembering hidden state

Prevent SetHiddenInTray() or Exit() from being called again after
qApp->quit() in MainWindow::closeEvent()

src/core/mainwindow.h

@@ -390,6 +390,7 @@ class MainWindow : public QMainWindow, public PlatformInterface {
   Song song_;
   Song song_playing_;
   AlbumCoverImageResult album_cover_;
+  bool exit_;
   int exit_count_;
   bool delete_files_;