Always re-add files to cache on reload (#1357)

The changes in this branch do make sure that a ThunarFile only will be released and removed from the cache after the last reference on it is dropped.

• No forced deletion of ThunarFiles anymore. Only send a destroy signal, so that users of the ThunarFile can stop using it and de-reference it.
• Don't remove a ThunarFile from the cache because the GFile cannot be loaded. Instead, just send a destroy signal, and wait until it properly got released by all other instances using it.

This fixes #1357 (closed), all integration tests do pass