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).

This excludes the second argument from template deduction.

Otherwise, it is required to manually cast the second argument to
the ConfigInfo type (because implicit conversions won't work).

e.g. to set the value for a ConfigInfo<std::string> from a string
literal, you'd need a ugly `std::string("yourstring")`.

Update Ubershader descriptions

The usual deduplication between DolphinWX and DolphinQt2, plus the removal
of a now unused ubershader string and a fix for the Euphoria i18n comment.

It's not common code that could be reused for, say, Citra;
it's absolutely specific to Wii emulation and only used by the Dolphin
core, so let's move it there.

Another reason for doing this is to avoid having Common depend on Core.