Config: Add support for enums

This makes it possible to use enums as the config type.
Default values are now clearer and there's no need for casts
when calling Config::Get/Set anymore.

In order to add support for enums, the common code was updated to
handle enums by using the underlying type when loading/saving settings.

A copy constructor is also provided for conversions from
`ConfigInfo<Enum>` to `ConfigInfo<underlying_type<Enum>>`
so that enum settings can still easily work with code that doesn't care
about the actual enum values (like Graphics{Choice,Radio} in DolphinQt2
which only treat the setting as an integer).

Source/Core/DolphinQt2/Config/Graphics/GraphicsChoice.h

@@ -6,11 +6,7 @@
 
 #include <QComboBox>
 
-namespace Config
-{
-template <typename T>
-struct ConfigInfo;
-}
+#include "Common/Config/Config.h"
 
 class GraphicsChoice : public QComboBox
 {
@@ -20,5 +16,5 @@ public:
 private:
   void Update(int choice);
 
-  const Config::ConfigInfo<int>& m_setting;
+  Config::ConfigInfo<int> m_setting;
 };