DolphinQt2: add DoubleClickEventFilter

This is an easy way to get a `doubleClicked` signal for any type
of widget without creating custom classes for each one.

Source/Core/DolphinQt2/QtUtils/DoubleClickEventFilter.cpp

@@ -0,0 +1,15 @@
+// Copyright 2017 Dolphin Emulator Project
+// Licensed under GPLv2+
+// Refer to the license.txt file included.
+
+#include <QEvent>
+
+#include "DolphinQt2/QtUtils/DoubleClickEventFilter.h"
+
+bool DoubleClickEventFilter::eventFilter(QObject* object, QEvent* event)
+{
+  if (event->type() == QEvent::MouseButtonDblClick)
+    emit doubleClicked();
+
+  return false;
+}

Source/Core/DolphinQt2/QtUtils/DoubleClickEventFilter.h

@@ -0,0 +1,17 @@
+// Copyright 2017 Dolphin Emulator Project
+// Licensed under GPLv2+
+// Refer to the license.txt file included.
+
+#pragma once
+
+#include <QObject>
+
+class DoubleClickEventFilter : public QObject
+{
+  Q_OBJECT
+signals:
+  void doubleClicked();
+
+private:
+  bool eventFilter(QObject* object, QEvent* event) override;
+};