Add the source application for drag and drop events (thanks Nathan!)

This is only implemented on iOS, but is useful for third party application integrations.

Fixes https://github.com/libsdl-org/SDL/issues/2024

src/core/android/SDL_android.c

@@ -878,7 +878,7 @@ JNIEXPORT void JNICALL SDL_JAVA_INTERFACE(onNativeDropFile)(
     jstring filename)
 {
     const char *path = (*env)->GetStringUTFChars(env, filename, NULL);
-    SDL_SendDropFile(NULL, path);
+    SDL_SendDropFile(NULL, NULL, path);
     (*env)->ReleaseStringUTFChars(env, filename, path);
     SDL_SendDropComplete(NULL);
 }

src/core/haiku/SDL_BeApp.cc

@@ -69,7 +69,7 @@ public:
         entry_ref entryRef;
         for (int32 i = 0; message->FindRef("refs", i, &entryRef) == B_OK; i++) {
             BPath referencePath = BPath(&entryRef);
-            SDL_SendDropFile(NULL, referencePath.Path());
+            SDL_SendDropFile(NULL, NULL, referencePath.Path());
         }
         return;
     }