Added SDL_SetAppMetadata() (#10404)

Removed duplicate hints SDL_HINT_APP_NAME, SDL_HINT_APP_ID, and
SDL_HINT_AUDIO_DEVICE_APP_NAME.

Wired up a few things to use the metadata; more to come!

Fixes https://github.com/libsdl-org/SDL/issues/4703

src/dynapi/SDL_dynapi.sym

@@ -158,6 +158,7 @@ SDL3_0.0.0 {
     SDL_GetAndroidInternalStoragePath;
     SDL_GetAndroidJNIEnv;
     SDL_GetAndroidSDKVersion;
+    SDL_GetAppMetadataProperty;
     SDL_GetAssertionHandler;
     SDL_GetAssertionReport;
     SDL_GetAudioDeviceChannelMap;
@@ -694,6 +695,8 @@ SDL3_0.0.0 {
     SDL_SendGamepadEffect;
     SDL_SendJoystickEffect;
     SDL_SendJoystickVirtualSensorData;
+    SDL_SetAppMetadata;
+    SDL_SetAppMetadataProperty;
     SDL_SetAssertionHandler;
     SDL_SetAudioDeviceGain;
     SDL_SetAudioPostmixCallback;