Add some support for letting other applications close your files on Vista

This is a new feature in Vista. Applications can allow other apps, including explorer.exe to close a file currently in use if for instance the user attempts to delete it while it is beeing edited. I'm not sure how much sense this makes for Qt, but perhaps an optional flag could be added.

See http://shellrevealed.com/blogs/shellblog/archive/2007/03/29/Your-File-Is-In-Use_2620_-Demystified.aspx for more details.