AudioPane: Fix WASAPI default device combo selection

Set the default value of `Config::MAIN_WASAPI_DEVICE` to `default`
instead of `Default`. This fixes an issue where `AudioPane`'s `Output
Device` combo would be blank if `WASAPI` was selected and the user had
never changed the value of `Output Device`.

We don't have to worry about backward compatibility with users who have
`Default` in their config because config values aren't written unless
they've been changed at some point from the default, and the combo has
always saved `default` instead of `Default`.

Audio still worked during emulation in this situation because the
fallback for an unrecognized device name is the default device.

To help prevent similar situations in the future references to
`MAIN_WASAPI_DEVICE`'s default value now call `GetDefaultValue` instead
of hardcoding the expected default, or use the new helper function
`Config::IsDefaultValue`.

Source/Core/UICommon/DiscordPresence.cpp

@@ -55,7 +55,7 @@ void HandleDiscordJoin(const char* join_secret)
   if (event_handler == nullptr)
     return;
 
-  if (Config::Get(Config::NETPLAY_NICKNAME) == Config::NETPLAY_NICKNAME.GetDefaultValue())
+  if (Config::IsDefaultValue(Config::NETPLAY_NICKNAME))
     Config::SetCurrent(Config::NETPLAY_NICKNAME, username);
 
   std::string secret(join_secret);