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/core/unix/SDL_appid.c

@@ -59,8 +59,7 @@ const char *SDL_GetExeName(void)
 
 const char *SDL_GetAppID(void)
 {
-    /* Always check the hint, as it may have changed */
-    const char *id_str = SDL_GetHint(SDL_HINT_APP_ID);
+    const char *id_str = SDL_GetAppMetadataProperty(SDL_PROP_APP_METADATA_NAME_STRING);
 
     if (!id_str) {
         /* If the hint isn't set, try to use the application's executable name */