Handle when the 'hidden' attribute changes

[dremin]

Currently the ShellFolders implementation does not properly handle hidden files:

• If a new hidden file is created, we don't check this and will still add the file to the collection, so it will be visible
• If a hidden file is renamed or modified, we will see that the file is not in the collection and add it, so it becomes visible
• If a visible file becomes hidden, the ChangeWatcher does not receive a notification of this and it remains visible

This PR fixes all of the above items.