DolphinQt2: Experimental GUI Warning Prompt.

Add a GUI prompt to tell users not to use DolphinQt by accident.

Source/Core/DolphinQt2/InDevelopmentWarning.h

@@ -0,0 +1,16 @@
+// Copyright 2016 Dolphin Emulator Project
+// Licensed under GPLv2+
+// Refer to the license.txt file included.
+
+#pragma once
+
+#include <QDialog>
+
+class InDevelopmentWarning final : public QDialog
+{
+  Q_OBJECT
+
+public:
+  explicit InDevelopmentWarning(QWidget* parent = nullptr);
+  ~InDevelopmentWarning();
+};