Make owned filename a pointer-to-non-const

The file handler owns the filename string, so it needs to free it.
Therefore, it should not be a pointer-to-const.

app/src/file_handler.h

@@ -46,9 +46,10 @@ file_handler_stop(struct file_handler *file_handler);
 void
 file_handler_join(struct file_handler *file_handler);
 
+// take ownership of file, and will SDL_free() it
 bool
 file_handler_request(struct file_handler *file_handler,
                      file_handler_action_t action,
-                     const char *file);
+                     char *file);
 
 #endif