Don't call g_object_ref/unref on objects that are being finalized.

ThunarFile objects were not being removed from the file_cache until
thunar_file_finalize().  This leaves a small window of time in which
another thread might call thunar_file_get(), and fetch the partially
finalized ThunarFile from the cache.

There are likely some remaining thread-safety issues with ThunarFile,
due to background jobs making changes to the same objects that are used
simultaneously in the UI thread.