Fix opening of project files on macOS (#3219)

- Double-click mmpz file in finder will now open properly on macOS
- Adds new MainApplication class for listening to QEvent::FileOpenEvent
- Queues open events until the application is ready to recieve them
- Closes #665

src/core/main.cpp

@@ -56,6 +56,7 @@
 
 #include <signal.h>
 
+#include "MainApplication.h"
 #include "MemoryManager.h"
 #include "ConfigManager.h"
 #include "NotePlayHandle.h"
@@ -255,7 +256,7 @@ int main( int argc, char * * argv )
 
 	QCoreApplication * app = coreOnly ?
 			new QCoreApplication( argc, argv ) :
-					new QApplication( argc, argv ) ;
+					new MainApplication( argc, argv );
 
 	Mixer::qualitySettings qs( Mixer::qualitySettings::Mode_HighQuality );
 	ProjectRenderer::OutputSettings os( 44100, false, 160,
@@ -834,6 +835,12 @@ int main( int argc, char * * argv )
 			gui->mainWindow()->showMaximized();
 		}
 
+		// Handle macOS-style FileOpen QEvents
+		QString queuedFile = static_cast<MainApplication *>( app )->queuedFile();
+		if ( !queuedFile.isEmpty() ) {
+			fileToLoad = queuedFile;
+		}
+
 		if( !fileToLoad.isEmpty() )
 		{
 			if( fileToLoad == recoveryFile )