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`.

Yellow squiggly lines begone!
Done automatically on .cpp files through `run-clang-tidy`, with manual corrections to the mistakes.
If an import is directly used, but is technically unnecessary since it's recursively imported by something else, it is *not* removed.
The tool doesn't touch .h files, so I did some of them by hand while fixing errors due to old recursive imports.
Not everything is removed, but the cleanup should be substantial enough.
Because this done on Linux, code that isn't used on it is mostly untouched.
(Hopefully no open PR is depending on these imports...)

If Rich Presence and Discord Presence are enabled in Achievement Settings, the string generated by rcheevos as the player's current Rich Presence will be sent to the Status field in the Discord Presence object. This will be updated whenever Rich Presence updates.

/home/ports/pobj/dolphin-5.0.0.20230429/dolphin-5.0.0.20230429/Source/Core/UICommon/DiscordPresence.cpp:27:13: warning: unused variable 's_using_custom_client' [-Wunused-variable]
static bool s_using_custom_client = false;
            ^

Otherwise, files that include the header get warning: ‘Discord::s_using_custom_client’ defined but not used.