Fix opening of project files on macOS

Per LMMS#665

include/EventApp.h

@@ -0,0 +1,9 @@
+#include <QApplication>
+
+class EventApp : public QApplication
+{
+public:
+	EventApp(int& argc, char * * argv);
+	bool event(QEvent *event);
+};
+

src/core/main.cpp

@@ -38,6 +38,7 @@
 #include <QMessageBox>
 #include <QPushButton>
 #include <QTextStream>
+#include <QFileOpenEvent>
 
 #ifdef LMMS_BUILD_WIN32
 #include <windows.h>
@@ -74,6 +75,7 @@
 #include "DataFile.h"
 #include "Song.h"
 #include "SetupDialog.h"
+#include "EventApp.h"
 
 static inline QString baseName( const QString & file )
 {
@@ -261,7 +263,7 @@ int main( int argc, char * * argv )
 
 	QCoreApplication * app = coreOnly ?
 			new QCoreApplication( argc, argv ) :
-					new QApplication( argc, argv ) ;
+					new EventApp( argc, argv ) ;
 
 	Mixer::qualitySettings qs( Mixer::qualitySettings::Mode_HighQuality );
 	ProjectRenderer::OutputSettings os( 44100, false, 160,
@@ -917,3 +919,30 @@ int main( int argc, char * * argv )
 
 	return ret;
 }
+
+
+
+
+EventApp::EventApp( int& argc, char * * argv ) : QApplication( argc, argv ) {}
+
+
+
+
+// Responds to QApplication events such as QFileOpenEvent (MacOS)
+bool EventApp::event( QEvent * event )
+{
+	switch(event->type())
+	{
+		case QEvent::FileOpen:
+		{
+			QFileOpenEvent * fileEvent = static_cast<QFileOpenEvent *>( event );
+			if( event )
+			{
+				Engine::getSong()->loadProject( fileEvent->file() );
+				return true;
+			}
+		}
+		default:
+			return QApplication::event( event );
+    }
+}