This pr adds an OnScreenDisplay message when you use the Volume hotkeys.
Just to have visual feedback.

Given this is actually a part of the Host interface, this should be
placed with it.

While we're at it, turn it into an enum class so that we don't dump its
contained values into the surrounding scope. We can also make
Host_Message take the enum type itself directly instead of taking a
general int value.

After this, it'll be trivial to divide out the rest of Common.h and
remove the header from the repository entirely

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")`.

Xlib has really terrible headers that declare non-namespaced
macros and typedefs for common words.

Just wasted 10 minutes trying to figure out why a unit test failed
to build before I remembered it was Xrandr.h conflicting with our
enum class members again.

To fix the issue, this removes the Display* parameter from the
EnableScreensaver function (which was unused) so we don't have
to include Xrandr.h anymore.

This means that any error messages won't be hidden by the fullscreen
window.