MainWindow: stop playing song when asking whether to save project

Users reported the last buffer played repeatedly while the dialog asking
whether to save project is shown. This behaviour even continued while
loading a project. An easy workaround for this is to stop playing song
(or whichever TrackContainer is active at the moment) in
MainWindow::mayChangeProject().

Closes bug #2904781.
(cherry picked from commit 04699ba412)

src/gui/MainWindow.cpp

@@ -548,6 +548,8 @@ void MainWindow::resetWindowTitle( void )
 
 bool MainWindow::mayChangeProject( void )
 {
+	engine::getSong()->stop();
+
 	if( !engine::getSong()->isModified() )
 	{
 		return( TRUE );