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/DolphinQt/Settings/AudioPane.cpp

@@ -123,8 +123,9 @@ void AudioPane::CreateWidgets()
 
 #ifdef _WIN32
   std::vector<std::pair<QString, QString>> wasapi_options;
-  wasapi_options.push_back(
-      std::pair<QString, QString>{tr("Default Device"), QStringLiteral("default")});
+  const auto default_device_config_value =
+      QString::fromStdString(Config::MAIN_WASAPI_DEVICE.GetDefaultValue());
+  wasapi_options.emplace_back(tr("Default Device"), default_device_config_value);
 
   for (auto string : WASAPIStream::GetAvailableDevices())
   {