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/DolphinWX/VideoConfigDiag.cpp

@@ -465,7 +465,8 @@ VideoConfigDiag::VideoConfigDiag(wxWindow* parent, const std::string& title)
         {
           szr_shader_compilation->Add(
               CreateRadioButton(page_general, modes[i].first, modes[i].second,
-                                Config::GFX_SHADER_COMPILATION_MODE, static_cast<int>(i)),
+                                Config::GFX_SHADER_COMPILATION_MODE,
+                                static_cast<ShaderCompilationMode>(i)),
               wxGBPosition(static_cast<int>(i / 2), static_cast<int>(i % 2)), wxDefaultSpan,
               wxALIGN_CENTER_VERTICAL);
         }