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/Common/Config/ConfigInfo.h

@@ -5,11 +5,19 @@
 #pragma once
 
 #include <string>
+#include <type_traits>
 
 #include "Common/Config/Enums.h"
 
 namespace Config
 {
+namespace detail
+{
+// std::underlying_type may only be used with enum types, so make sure T is an enum type first.
+template <typename T>
+using UnderlyingType = typename std::enable_if_t<std::is_enum<T>{}, std::underlying_type<T>>::type;
+}  // namespace detail
+
 struct ConfigLocation
 {
   System system;
@@ -24,6 +32,21 @@ struct ConfigLocation
 template <typename T>
 struct ConfigInfo
 {
+  ConfigInfo(const ConfigLocation& location_, const T& default_value_)
+      : location{location_}, default_value{default_value_}
+  {
+  }
+
+  // Make it easy to convert ConfigInfo<Enum> into ConfigInfo<UnderlyingType<Enum>>
+  // so that enum settings can still easily work with code that doesn't care about the enum values.
+  template <typename Enum,
+            std::enable_if_t<std::is_same<T, detail::UnderlyingType<Enum>>::value>* = nullptr>
+  ConfigInfo(const ConfigInfo<Enum>& other)
+      : location{other.location}, default_value{static_cast<detail::UnderlyingType<Enum>>(
+                                      other.default_value)}
+  {
+  }
+
   ConfigLocation location;
   T default_value;
 };

Source/Core/Common/Config/Layer.h

@@ -8,6 +8,7 @@
 #include <memory>
 #include <optional>
 #include <string>
+#include <type_traits>
 #include <vector>
 
 #include "Common/Config/ConfigInfo.h"
@@ -25,8 +26,13 @@ std::string ValueToString(double value);
 std::string ValueToString(int value);
 std::string ValueToString(bool value);
 std::string ValueToString(const std::string& value);
+template <typename T, std::enable_if_t<std::is_enum<T>::value>* = nullptr>
+std::string ValueToString(T value)
+{
+  return ValueToString(static_cast<std::underlying_type_t<T>>(value));
+}
 
-template <typename T>
+template <typename T, std::enable_if_t<!std::is_enum<T>::value>* = nullptr>
 std::optional<T> TryParse(const std::string& str_value)
 {
   T value;
@@ -35,6 +41,15 @@ std::optional<T> TryParse(const std::string& str_value)
   return value;
 }
 
+template <typename T, std::enable_if_t<std::is_enum<T>::value>* = nullptr>
+std::optional<T> TryParse(const std::string& str_value)
+{
+  const auto result = TryParse<std::underlying_type_t<T>>(str_value);
+  if (result)
+    return static_cast<T>(*result);
+  return {};
+}
+
 template <>
 inline std::optional<std::string> TryParse(const std::string& str_value)
 {