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/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)
 {