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/gui/MainApplication.cpp

@@ -0,0 +1,68 @@
+/*
+ * MainApplication.cpp - Main QApplication handler
+ *
+ * Copyright (c) 2017-2017 Tres Finocchiaro <tres.finocchiaro/at/gmail.com>
+ *
+ * This file is part of LMMS - http://lmms.io
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public
+ * License as published by the Free Software Foundation; either
+ * version 2 of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public
+ * License along with this program (see COPYING); if not, write to the
+ * Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
+ * Boston, MA 02110-1301 USA.
+ *
+ */
+#include "MainApplication.h"
+
+#include <QDebug>
+#include <QFileOpenEvent>
+
+#include "Engine.h"
+#include "GuiApplication.h"
+#include "MainWindow.h"
+#include "Song.h"
+
+MainApplication::MainApplication(int& argc, char** argv) :
+	QApplication(argc, argv),
+	m_queuedFile() {}
+
+bool MainApplication::event(QEvent* event)
+{
+	switch(event->type())
+	{
+		case QEvent::FileOpen:
+		{
+			QFileOpenEvent * fileEvent = static_cast<QFileOpenEvent *>(event);
+			if(event)
+			{
+				// Handle the project file
+				m_queuedFile = fileEvent->file();
+				if(Engine::getSong())
+				{
+					if(gui->mainWindow()->mayChangeProject(true))
+					{
+						qDebug() << "Loading file " << m_queuedFile;
+						Engine::getSong()->loadProject(m_queuedFile);
+					}
+				}
+				else
+				{
+					qDebug() << "Queuing file " << m_queuedFile;
+				}
+				return true;
+			}
+			// Fall through to default
+		}
+		default:
+			return QApplication::event(event);
+	}
+}