Implement Android intent handling in qml app

AndroidManifest.xml register activity as an handler for retroshare links
RetroShareQmlActivity...NativeCalls.cpp bring the intent data from java
  to C++ and then to QML
QMl mainWindow uses URI.js to parse the received uri
Create a singleton for qml engine so it is reachable from NativeCalls

retroshare-qml-app/src/singletonqmlengine.h

@@ -0,0 +1,32 @@
+#pragma once
+/*
+ * libresapi local socket client
+ * Copyright (C) 2017  Gioacchino Mazzurco <gio@eigenlab.org>
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as
+ * published by the Free Software Foundation, either version 3 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 Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include <QQmlApplicationEngine>
+
+struct SingletonQmlEngine
+{
+	static QQmlApplicationEngine& instance()
+	{
+		static QQmlApplicationEngine engine;
+		return engine;
+	}
+
+private:
+	SingletonQmlEngine();
+};