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/CMakeLists.txt

@@ -40,6 +40,7 @@ set(SRCS
   GameList/GameListModel.cpp
   GameList/GameTracker.cpp
   GameList/ListProxyModel.cpp
+  QtUtils/DoubleClickEventFilter.cpp
   QtUtils/ElidedButton.cpp
   Settings/GeneralPane.cpp
   Settings/InterfacePane.cpp