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

include/MainApplication.h

@@ -0,0 +1,43 @@
+/*
+ * MainApplication.h - 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.
+ *
+ */
+
+#ifndef MAINAPPLICATION_H
+#define MAINAPPLICATION_H
+
+#include <QApplication>
+
+class MainApplication : public QApplication
+{
+public:
+	MainApplication(int& argc, char** argv);
+	bool event(QEvent* event);
+	inline QString& queuedFile()
+	{
+	    return m_queuedFile;
+	}
+private:
+	QString m_queuedFile;
+};
+
+#endif // MAINAPPLICATION_H